DOI: 10.1016/j.cplett.2024.141782  (OpenAccess)

Unexpected Enhanced acidity of Diborane-Nitrogen Base complexes

The effect that borane and diborane have on the structure and acidity of various N-bases was analyzed using G4 ab-initio calculations. Although diborane is expected to be a stronger Lewis acid than borane, its interaction with N-bases is weaker due to the loss of one of its (3c,2e) bonds. Surprisingly, however, diborane has a significantly greater effect on the intrinsic acidity of these bases than borane. Upon deprotonation of the base, the interaction strength increases by a factor of 10 with diborane and only 3 with borane, as both the B–N and BH3···BH3 interactions are reinforced in diborane.

DOI: 10.1002/jcc.27509  (OpenAccess)

Hydride and halide abstraction reactions behind the enhanced basicity of Be and Mg clusters with nitrogen bases

In this study, we investigate the protonation effects on the structure, relative stability and basicity of complexes formed by the interaction of monomers and dimers of BeX2 and MgX2 (X = H, F) with NH3, CH2NH, HCN, and NC5H5 bases. Calculations were performed using the M06-2X/aug-cc-pVTZ formalism, along with QTAIM, ELF and NCI methods for electron density analysis and MBIE and LMO-EDA energy decomposition analyses for interaction enthalpies. The protonation of the MH2– and M2H4–Base complexes occurs at the negatively charged hydrogen atoms of the MH2 and M2H4 moieties through typical hydride abstraction reactions, while protonation at the N atom of the base is systematically less exothermic. The preference for the hydride transfer mechanism is directly associated with the significant exothermicity of H2 formation through the interaction between H− and H+, and the high hydride donor ability of these complexes. The basicity of both, MH2 and M2H4 compounds increases enormously upon association with the corresponding bases, with the increase exceeding 40 orders of magnitude in terms of ionization constants. Due to the smaller exothermicity of HF formation, the basicity of fluorides is lower than that of hydrides. In Be complexes, the protonation at the N atom of the base dominates over the fluoride abstraction mechanism. However, for the Mg complexes the fluoride abstraction mechanism is energetically the most favorable process, reflecting the greater facility of Mg complexes to lose F−.

DOI: 10.1039/D4CP03894K  (OpenAccess)

Simple carbenes as hydrogen bond acceptors: ab initio determination of nucleophilicities and reduced nucleophilicities

Nucleophilicities for a range of simple carbene molecules acting as hydrogen bond acceptors B in forming complexes B⋯HX are reported. The carbenes chosen to fulfil the roles of a Lewis base are B = R2M, cyclo-(CH)2M, H2CCM and two N-heterocyclic carbenes, where M is one of the group 14 tetrel atoms, C, Si, Ge or Sn and R = H, CH3, and F. All the carbenes but CH2 have a singlet electronic ground state. The Lewis acids, HX, involved are HF, HCl, HBr, HI and HCN, HCCH, and HCP. Nucleophilicities, NB, of the carbenes were determined graphically from equilibrium dissociation energies, De, for the process B⋯HX = B + HX by using the equation De = c·NB·EHX, where c = 1.0 kJ mol−1 and the EHX are known numerical electrophilicities of the Lewis acids HX. De values were calculated ab initio at the CCSD(T)-F12c/cc-pVDZ-F12 level of theory, which for CH2 refers to the singlet electronic excited state. It was established that NR2M values lie in the order of M = C ≫ Si ∼ Ge ∼ Sn for a given R and in the order R = CH3 > H > F for a given M. Reduced nucleophilicities, NB/σaxial, were determined by using the molecular electronic surface potential σaxial at atom M (which lies on the C2 axis) on the 0.001 e Bohr−3 iso-surface of each carbene molecule, as calculated at the MP2/aug-cc-pVTZ level. For R2M having R = CH3 and H and cyclo-(CH)2-M carbenes, the determined values of NB/σaxial are shown to be independent of R and M.

DOI: 10.1002/cphc.202400608   (OpenAccess)

Beryllium as a Base: Complexes of Be(CO)₃ with HX (X=F, Cl, Br, CN, NC, CCH, OH)

We have explored the complexes that can be formed between XH hydrogen bond donors and Be(CO)₃, an unusual base where Be inverts its typical Lewis acid character and can act as a base towards hydrogen bonding with a strength similar to that of pi systems.

DOI: 10.3762/bjoc.20.224  (OpenAccess)

Computational design for enantioselective CO₂ capture: asymmetric frustrated Lewis pairs in epoxide transformations

Carbon capture and utilisation (CCU) technologies offer a compelling strategy to mitigate rising atmospheric carbon dioxide levels. Despite extensive research on the CO₂ insertion into epoxides to form cyclic carbonates, the stereochemical implications of this reaction have been largely overlooked, despite the prevalence of racemic epoxide solutions. This study introduces an in silico approach to design asymmetric frustrated Lewis pairs (FLPs) aimed at controlling reaction stereochemistry. Four FLP scaffolds, incorporating diverse Lewis acids (LA), Lewis bases (LB), and substituents, were assessed via volcano plot analysis to identify the most promising catalysts. By strategically modifying LB substituents to induce asymmetry, a stereoselective catalytic scaffold was developed, favouring one enantiomer from both epoxide enantiomers. This work advances the in silico design of FLPs, highlighting their potential as asymmetric CCU catalysts with implications for optimising catalyst efficiency and selectivity in sustainable chemistry applications.

DOI: 10.1002/cphc.202000099  (OpenAccess)

Computational Insights into Cinchona-Based PhaseTransfer Catalysis for Asymmetric Conjugate Cyanation

Computational insights into the conjugate cyanation of α,β-unsaturated ketones using quinine-derived phase transfer catalysts are presented. The study examines different binding modes, reaction mechanisms, and non-covalent interactions, demonstrating how catalyst modifications enhance enantioselectivity and invert product configuration.

DOI: 10.1002/cplu.202400314  (OpenAccess)

Monohydrides of the Group 13 Elements M=B, Al and Ga: Axial Bi-Nucleophilicity and the Propensity to Form Both H-M...HX and M-H...HX Hydrogen Bonds (X=F, Cl, Br, I, CN, CCH, CP)

Nucleophilicities NH−M of H−M (M=Al or Ga) when forming H-bond complexes with HX are the gradients. Both H and M ends of M−H are nucleophilic and can form H-bond complexes, with NH end>NM end . The order of NH−M is Ga ~Al when atom M is the H-bond acceptor and also Ga ~ Al when H is the acceptor.

DOI: 10.1002/cphc.202400308  (OpenAccess)

Low Valence Triel (I) Systems as Hydrogen Bond Acceptors and their Stability with Respect to Triel (III) Compounds

Low valence carbene like triel (I) systems can act as hydrogen bond acceptors creating complexes that can evolve into triel (III) adducts. The stability of those is studied and the nature of the interaction analysed, being charge transfer the main driven force. Computational tools like NBO, EDS, QTAIM results corroborate those findings.

DOI: 10.1021/acs.jpca.4c03438  (OpenAccess)

Ab Initio Investigation of Tetrel Bonds in Isolated Complexes Formed Between a Lewis Acid H3MX, M–O or M–S (M = Si, Ge, or Sn) and the Lewis Bases B = N2, CO, HCCH, PH3, C2H4, HCN, CS, HNC, NP, H2O, and NH3

Isolated complexes of the type B⋯A in which the noncovalent interaction is a tetrel bond have been characterized by ab initio calculations at the CCSD(T)(F12c)/cc-pVDZ-F12 level. The Lewis bases B involved were N2, CO, HCCH, PH3, C2H4, HCN, CS, HNC, NP, H2O and NH3. Two types of Lewis acid A were examined, each containing one of the tetrel atoms M = Si, Ge or Sn, The Lewis acids in the first series were the H3MX (X = F, Cl, CN, H), in each of which the most electrophilic region was found to lie on the C3 axis of the C3v molecules, near to the tetrel atom M. In the second series the Lewis acids were M–O and M–S. Graphs, consisting of calculated equilibrium dissociation energies De of each B⋯H3MX complex plotted against the nucleophilicities NB of the Lewis bases B, were used to obtain the electrophilicity EH3MX of each molecule H3MX (M = Si, Ge, Sn). The molecular electrostatic surface of potentials of the molecules M–S and M–O (M = Si, Ge, Sn) revealed that many of the B⋯M-S and B⋯M−O complexes should have a tetrel bond to M in which the axis of the M–S or M–O subunit should be approximately perpendicular to the axis of the nonbonding or π-bonding electron pair carried by B, a novel type of tetrel bond confirmed by geometry optimizations of the complexes.

DOI: 10.1002/cphc.202000099   (OpenAccess)

Discovering trends in the Lewis acidity of beryllium and magnesium hydrides and fluorides with increasing clusters size

We have reported in the last years the strong effect that Be- and Mg-containing Lewis acids have on the intrinsic properties of typical bases, which become acids upon complexation. In an effort to investigate these changes when the Be and Mg derivatives form clusters of increasing size, we have examined the behavior of the (MX2)n (M = Be, Mg; X = H, F; n = 1, 2, 3) clusters when they interact with ammonia, methanimine, hydrogen cyanide and pyridine, and with their corresponding deprotonated forms. The complexes obtained at the M06-2X/aug-cc-pVTZ level were analyzed using the MBIE energy decomposition formalism, in parallel with QTAIM, ELF, NCIPLOT and AdNDP analyses of their electron density. For n = 1 the interaction enthalpy for the different families of monomers, Be (Mg) hydrides and Be (Mg) fluorides, follows the same trend as the intrinsic basicity of the base that interacts with them. This interaction is greatly reinforced after the deprotonation of the base, resulting in a significant enhancement of the intrinsic acidity of the corresponding MX2–Base complex. For (MX2)2 clusters a further reinforcement of the interaction with the base is observed, this reinforcement being again larger for the deprotonated complexes. However, the concomitant increase of their intrinsic acidity is one order of magnitude larger for hydrides than for fluorides. Unexpectedly, the cyclic conformers (MX2)3, which are more unstable than the linear ones, become the global minima after association with the base and the same is true for the deprotonated complex. Accordingly, a further increase of the intrinsic acidity of the (MX2)3–Base complexes with respect to the (MX2)2–Base ones is observed. This effect is maximum for (MgF2)3 clusters, to the point that the (MgF2)3–Base complexes become more acidic than nitric acid, the extreme case being the cluster (MgF2)3–NCH, whose acidity is higher than that of perchloric acid.

DOI: 10.1007/s00894-024-05992-3  (OpenAccess)

Understanding the coupling of non‑metallic heteroatoms to CO2 from a Conceptual DFT perspective

A Conceptual DFT (CDFT) study has been carry out to analyse the coupling reactions of the simplest amine (CH₃NH₂), alcohol (CH₃OH), and thiol (CH₃SH) compounds with CO₂ to form the corresponding adducts CH₃NHCO₂H, CH₃OCO₂H, and CH₃SCO₂H. The reaction mechanism takes place in a single step comprising two chemical events: nucleophilic attack of the non-metallic heteroatoms to CO₂ followed by hydrogen atom transfer (HAT). According to our calculations, the participation of an additional nucleophilic molecule as HAT assistant entails important decreases in activation electronic energies. In such cases, the formation of a six-membered ring in the transition state (TS) reduces the angular stress with respect to the non-assisted paths, characterised by four-membered ring TSs. Through the analysis of the energy and reaction force profiles along the intrinsic reaction coordinate (IRC), the ratio of structural reorganisation and electronic rearrangement for both activation and relaxation energies has been computed. In addition, the analysis of the electronic chemical potential and reaction electronic flux profiles confirms that the highest electronic activity as well as their changes take place in the TS region. Finally, the distortion/interaction model using an energy decomposition scheme based on the electron density along the reaction coordinate has been carried out and the relative energy gradient (REG) method has been applied to identify the most important components associated to the barriers.

DOI: 10.1039/d4cp00496e   (OpenAccess)

A multi-FLP approach for CO2 capture: investigating nitrogen, boron, phosphorus and aluminium doped nanographenes and the influence of a sodium cation

The reactivity of B3N3-doped hexa-cata-hexabenzocoronene (B3N3-NG), Al3N3-NG, B3P3-NG and Al3P3-NG, models of doped nanographenes (NGs), towards carbon dioxide was studied with density functional theory (DFT) calculations at the M06-2X/6-311++G(3df,3pd)//M06-2X/6-31+G* level of theory. The NG systems exhibit a poly-cyclic poly-frustrated Lewis pair (FLP) nature, featuring multiple Lewis acid/Lewis base pairs on their surface enabling the capture of several CO2 molecules. The capture of CO2 by these systems was investigated within two scenarios: (A) sequential capture of up to three CO2 molecules and (B) capture of CO2 molecules in the presence of a sodium cation. The resulting adducts were analyzed in terms of the activation barriers and relative stabilities. The presence of aluminium atoms changes the asynchrony of the reaction favoring the aluminium-oxygen bond and influences the regioselectivity of the multi-capture. A cooperative effect is predicted due to π-electron delocalization, with the sodium cation stabilizing the stationary points and favoring the addition of CO2 to the NGs.

DOI: 10.1021/acs.jpclett.4c00779   (OpenAccess)

Hydrogen Bonds Are Never of an “Anti-electrostatic” Nature: A Brief Tour of a Misleading Nomenclature

A large amount of scientific works have contributed through the years to rigorously reflect the different forces leading to the formation of hydrogen bonds, the electrostatic and polarization ones being the most important among them. However, we have witnessed lately with the emergence of a new terminology, anti-electrostatic hydrogen bonds (AEHBs), that seems to contradict this reality. This nomenclature is used in the literature to describe hydrogen bonds between equally charged systems to justify the existence of these species, despite numerous proofs showing that AEHBs are, as any other hydrogen bond between neutral species, mostly due to electrostatic forces. In this Viewpoint, we summarize the state of the art regarding this issue, try to explain why this terminology is very misleading, and strongly recommend avoiding its use based on the hydrogen bond physical grounds.

DOI: 10.1007/s11224-024-02286-5   (OpenAccess)

Theoretical study of the formation of pyrazole and indazole carbamic acids

A theoretical study of the formation of carbamic acids of pyrazole and indazole has been carried out using DFT computational methods. The effects of the substituents and the solvent (using explicit and implicit solvent models) have been considered. In addition, the deprotonation of the carbamic acid and its influence on the stability of the system has been calculated. In the neutral systems, only the formation of indazole-1-carbamic acid derivatives is favored vs. the non-covalent complexes between pyrazole or indazole with CO2. The deprotonation of the carbamic acid highly stabilizes the system preventing its dissociation.

DOI: 10.1021/acs.jpca.3c08412  (OpenAccess)

Capture of CO2 by Melamine Derivatives: A DFT Study Combining the Relative Energy Gradient Method with an Interaction Energy Partitioning Scheme

A theoretical study of the interaction between melamine and CO2 was carried out using density functional theory (DFT) with the B3LYP-D3(BJ)/aug-cc-pVTZ level of theory. The presence of anions interacting with melamine transforms the weakly bonded tetrel complexes into adducts. Thus, melamine acts as an FLP (frustrated Lewis pair) with acid groups (NHs as hydrogen bond donors) and a base group (N of the triazine ring). The application of the relative energy gradient formalism (REG) along the reaction coordinate has demonstrated that the ability of the melamine-anion systems to capture CO2 is linked to its capacity to polarize the CO2 molecule. These results have been confirmed by placing the melamine:CO2 complex in a uniform electric field with different strengths.

DOI: 10.1002/cphc.202300750  (OpenAccess)

(Pyridin-2-ylmethyl)triel Derivatives as Masked Frustrated Lewis Pairs. Interactions and CO₂-Sequestration

The isolated (pyridin-2-ylmethyl)triel derivatives (triel=B, Al and Ga) show an intramolecular N⋅⋅⋅Tr triel bond. Their reaction with CO₂ indicate that they can be classified as masked frustrated Lewis pairs (mFLP). The adduct formation proceeds in two steps: breaking the intramolecular N⋅⋅⋅Tr bond and adduct formation with Tr−O and N−C covalent bonds, being final products always more stable.

DOI: 10.3390/molecules28227507   (OpenAccess)

A Holistic View of the Interactions between Electron-Deficient Systems: Clustering of Beryllium and Magnesium Hydrides and Halides

 

DOI: 10.1016/j.cplett.2023.140929  (OpenAccess)

Nucleophilicities of the simplest saturated and unsaturated hydrocarbons

Nucleophilicities NB of prototype saturated and unsaturated hydrocarbons methane, ethane, cyclopropane, ethyne and ethene when acting as Lewis bases were determined from the dissociation energies De of the hydrogen-bond complexes B…HX calculated ab initio, where B is the hydrocarbon and HX is variously HF, HCl, HBr, HI, HCN, HCCH and HCP. The values of NB obtained were 0.712(31), 0.736(32), 0.283(44), 2.561(73), 2.604(51) for methane, ethane, cyclopropane, ethyne and ethene, respectively. The inductive effect of the methyl group when ethyne is methylated was also investigated.

DOI: 10.1021/acs.inorgchem.3c02355

Metastable Charged Dimers in Organometallic Species: A Look into Hydrogen Bonding between Metallocene Derivatives

Multiply charged complexes bound by noncovalent interactions have been previously described in the literature, although they were mostly focused on organic and main group inorganic systems. In this work, we show that similar complexes can also be found for organometallic systems containing transition metals and deepen in the reasons behind the existence of these species. We have studied the structures, binding energies, and dissociation profiles in the gas phase of a series of charged hydrogen-bonded dimers of metallocene (Ru, Co, Rh, and Mn) derivatives isoelectronic with the ferrocene dimer. Our results indicate that the carboxylic acid-containing dimers are more strongly bonded and present larger barriers to dissociation than the amide ones and that the cationic complexes tend to be more stable than the anionic ones. Additionally, we describe for the first time the symmetric proton transfer that can occur while in the metastable phase. Finally, we use a density-based energy decomposition analysis to shine light on the nature of the interaction between the dimers.

DOI: 10.1002/cphc.202300214  (OpenAccess)

Production of Dihydrogen Using Ammonia Borane as Reagent and Pyrazole as Catalyst

Theoretical chemistry (DLPNO-CCSD(T)/def2-TZVP//M06-2x/aug-cc-pVDZ) was used to design a system based on ammonia boranes catalyzed by pyrazoles with the aim of producing dihydrogen, nowadays of high interest as clean fuel. The reactivity of ammonia borane and cyclotriborazane were investigated, including catalytic activation through 1H-pyrazole, 4-methoxy-1H-pyrazole, and 4-nitro-1H-pyrazole. The results point toward a catalytic cycle by which, at the same time, ammonia borane can initially store and then, through catalysis, produce dihydrogen and amino borane. Subsequently, amino borane can trimerize to form cyclotriborazane that, in presence of the same catalyst, can also produce dihydrogen. This study proposes therefore a consistent progress in using environmentally sustainable (metal free) catalysts to efficiently extract dihydrogen from small B−N bonded molecules.

DOI: 10.1007/s00214-023-03013-9  (OpenAccess)

Influence of Lewis acids on the symmetric SN2 reaction

This paper presents a theoretical analysis the effect of non-covalent interactions (NCI) in three different SN2 reactions (X–:CH3X → XCH3:X–, X = Cl, Br and I) has been theoretically analysed in the pre-reactive complexes, TS and products. A total of eighteen Lewis acids (LAs: FH, ClH, FCl, I2, SeHF, SeF2, PH2F, PF3, SiH3F, SiF4, BH3, BF3, BeH2, BeF2, LiH, LiF, Au2 and AgCl) interact with the halogen atom of the CH3X molecule. To analyse the strength of the non-covalent interactions, both the independent gradient model tool and electron density maps have been employed. The results reveal that in all cases, the interaction between the anion and the Lewis acid leads to an increase in the transition barriers compared to the parental reaction.

DOI: 10.1007/s11224-023-02170-8  (OpenAccess)

Borane derivatives of five-membered N-heterocyclic rings as frustrated Lewis pairs: activation of CO2

The reaction of seventeen borane derivatives of five-membered N-heterocyclic rings (BNHRs) with CO2 has been studied by means of DFT calculations. Several non-covalent complexes between the BNHRs and CO2 which evolve through a TS in a single adduct for each BNHR have been identified. The calculated IRC of the TS has allowed to identify the non-covalent complex involved in the reaction in each case. The stationary points of the reactions have been analyzed with the distortion/interaction partition model. In addition, empirical models have been attempted to correlate the acid (fluoride ion affinity) and basic (proton affinity) properties of the isolated BNHR with the TS barriers and adduct energies. The energetics of the reactions are influenced by the number of nitrogen atoms in the ring.

DOI: 10.1021/acs.jpca.3c02747   (OpenAccess)

Dispersion, Rehybridization, and Pentacoordination: Keys to Understand Clustering of Boron and Aluminum Hydrides and Halides

The structure, stability, and bonding characteristics of dimers and trimers involving BX3 and AlX3 (X = H, F, Cl) in the gas phase, many of them explored for the first time, were investigated using different DFT (B3LYP, B3LYP/D3BJ, and M06-2X) and ab initio (MP2 and G4) methods together with different energy decomposition formalisms, namely, many-body interaction-energy and localized molecular orbital energy decomposition analysis. The electron density of the clusters investigated was analyzed with QTAIM, electron localization function, NCIPLOT, and adaptive natural density partitioning approaches. Our results for triel hydride dimers and Al2X6 (X = F, Cl) clusters are in good agreement with previous studies in the literature, but in contrast with the general accepted idea that B2F6 and B2Cl6 do not exist, we have found that they are predicted to be weakly bound systems if dispersion interactions are conveniently accounted for in the theoretical schemes used. Dispersion interactions are also dominant in both homo- and heterotrimers involving boron halide monomers. Surprisingly, B3F9 and B3Cl9 C3v cyclic trimers, in spite of exhibiting rather strong B–X (X = F, Cl) interactions, were found to be unstable with respect to the isolated monomers due to the high energetic cost of the rehybridization of the B atom, which is larger than the two- and three-body stabilization contributions when the cyclic is formed. Another important feature is the enhanced stability of both homo- and heterotrimers in which Al is the central atom because Al is systematically pentacoordinated, whereas this is not the case when the central atom is B, which is only tri- or tetra-coordinated.

DOI: 10.1021/acs.iecr.3c00475 

Effect of Surface Organo-Silanization on SBA-15 Mesoporous Silicas in CO2 Adsorption Processes: Design, Synthesis, and Computational Studies

Carbon dioxide solid sorbents produced from mesoporous functionalized silica microparticles (SBA-15) have been investigated (i) theoretically using density functional theory and (ii) evaluated empirically for assessing their CO2 adsorption capacity. Two different families of organosilyl groups have been tested possessing a common anchoring group (silanol), in one extreme, but bearing two different types of CO2 sensitive groups in the other extreme; (i) hyperbranched polymeric PAMAM moieties, carrying multiple −NH2 groups, and (ii) a collection of linear functional ending groups such as −SH, −SO3H, −guanidine (Gdn), −NH2, −NCO, and −N3. The adsorption isotherms revealed that SBA-15 bearing (3-aminopropyl)triethoxysilane (APTES) showed an impressive 3.4-fold adsorption enhancement at 1 bar and 50 °C when compared to the pristine SBA-15, following a straightforward synthetic protocol. The maximum adsorption capacity was increased from 0.34 mmol/g (SBA-15) to 1.15 mmol/g (SBA-15@NH2) under conditions relevant to CO2 capture (1 bar and 50 °C). We also found intriguing certain discrepancies observed between the calculated CO2 isotherms and the theorized binding energy in two of the materials. This will be addressed in the present work.

DOI: 10.1080/00268976.2022.2086935  (OpenAccess)

On predicting bonding patterns of small clusters of alkaline-earth (Be, Mg) and triel (B, Al) fluorides: a balance between atomic size and electron-deficient character

The structures, bonding and stability of (MF2)m:(M´F3)n (M = Be, Mg; M´ =  B, Al; m = 0,1,2; n = 0,1,2) clusters were obtained at the B3LYP/aug-cc-pVTZ level of theory. To understand trends across this set of closely related atoms, an analysis of the results obtained using atoms in molecules (AIM), electron localisation function (ELF) and electron density shift (EDS) approaches, permits to identify subtle dissimilarities when the first-row elements, Be and B, are replaced by the second-row counterparts, Mg and Al. For dimers this replacement involves an increase in the bonding enthalpies as a direct consequence of a much larger ionic character of the derivatives including second-row elements. For trimers and tetramers, rather stable structures involving penta-coordinated aluminium atoms are formed, which are not found for the B-containing analogues. In all clusters investigated the electronic environment around each monomer does not change significantly neither with nature of the monomers interacting with it or with the size of the cluster, though some small cooperative effects are observed when analyzing the binding enthalpies. The important consequence is that the stability of larger clusters can be easily predicted through a statistical treatment of the values obtained for the smaller ones.

DOI: 10.1016/j.molstruc.2023.135936

Synthesis and characterization of two novel fused macrocyclic ligands: Experimental and theoretical studies

Two new 24- and 26-membered macrocycles (L1 and L2) have been prepared from condensation of N 1-[2-(2-aminophenoxy)ethyl]-benzene-1,2-diamine (1), N 1-(3-(2-aminophenoxy) propyl) benzene-1,2-diamine (2) and 2,6-diformyl-4-methylphenol in methanol. The ligands were characterized with various spectroscopic methods such as FT-IR, 1H and 13C NMR and ESI-Mass and elemental analysis. The structure of the L2 has been determined by single-crystal X-ray diffraction analysis. DFT studies at B3LYP/6-31+G(d,p)/GD3 computational level have been carried out and compared to the X-ray results.

DOI: 10.1021/acs.jpca.3c02159  (OpenAccess)

Hydrogen-Bond Dissociation Energies from the Properties of Isolated Monomers

The strength of binding, as measured by the equilibrium dissociation energy De of an isolated hydrogen-bonded complex B···HX, where B is a simple Lewis base and X = F, Cl, Br, I, CN, CCH, or CP, can be determined from the properties of the infinitely separated components B and HX. The properties in question are the maximum and minimum values σmax(HX) and σmin(B) of the molecular electrostatic surface potentials on the 0.001 e/bohr3 iso-surfaces of HX and B, respectively, and two recently defined quantities: the reduced electrophilicity ΞHX of HX and the reduced nucleophilicity ИB of B. It is shown that De is given by the expression De = {σmax(HX)σmin(B)} ИB ΞHX. This is tested by comparing De calculated ab initio at the CCSD(T)(F12c)/cc-pVDZ-F12 level of theory with that obtained from the equation. A large number of complexes (203) falling into four categories involving different types of hydrogen-bonded complex B···HX are investigated: those in which the hydrogen-bond acceptor atom of B is either oxygen or nitrogen, or carbon or boron. The comparison reveals that the proposed equation leads to De values in good agreement in general with those calculated ab initio.

DOI: 10.1002/cplu.202300032   (OpenAccess)

Reduced Nucleophilicities ИB of Lewis Bases B: Is ИB Independent of Whether B is Involved in a Hydrogen Bond or a Halogen Bond?

Reduced nucleophilicities ИB of axially symmetric molecules B were determined from , where De is the equilibrium dissociation energy of the complexes B⋅⋅⋅XY, NB is the nucleophilicity of B, EXY is the electrophilicity of the halogen-bond donor XY and  is the minimum electrostatic surface potential of B. The series B⋅⋅⋅ClY, B⋅⋅⋅BrY, B⋅⋅⋅IY (Y=F, Cl, Br, I, CN, and CCH) as well as (B⋅⋅⋅XY, XY=F2, Cl2, Br2,and BrCl) of complexes were investigated. Molecules B were grouped so that the terminal atom involved in the halogen bond was fixed within the group. Groups having N as the terminal atom were RCN (R=CH3, H, and F) or RN (R=N and P), those with C as the terminal atom were RNC (R=H and F) and RC (R=O, S and Se), and those with a terminal O atom were R=C=O (R=O or S). Graphs of  versus EXY for each group were straight lines through the origin, with generally different gradients, hence implying different NB. By contrast, when  was the ordinate the lines conflated to give a single straight line, which then defines a common (reduced) nucleophilicity ИB for that group of B. Hence it was concluded that ИB is an intrinsic property of the terminal atom, independent of the remainder of B, and only weakly dependent on the type (C, N or O) of the terminal atom. Moreover, ИB for each B was the same as determined previously from the hydrogen-bonded series B⋅⋅⋅HX, (X=F, Cl, Br, I, CN, CCH, and CP).

DOI: 10.1038/s41598-023-29336-y  OpenAccess

Reactivity of a model of B3P3 ‑doped nanographene with up to three CO₂ molecules

The reactivity of a B3P3-doped hexa-cata-hexabenzocoronene, as a model of nanographene (B3P3-NG), towards carbon dioxide was studied at the DFT M06-2X/6-311++G(3df,3pd)//M06-2X/6-31+G* level of theory. This compound can be classified as a poly-cyclic poly-Frustrated Lewis Pair (FLP) system, as it presents more than one Lewis Acid/Lewis Base pair on its surface, making the capture of several carbon dioxide molecules possible. Two scenarios were considered to fully characterize the capture of CO2 by this multi-FLP system: (i) fixation of three CO2 molecules sequentially one by one; and (ii) simultaneous contact of three CO₂ molecules with the B3P3-NG surface. The resulting adducts were analyzed as function of activation barriers and the relative stability of the CO₂ capture. A cooperativity effect due to the π-delocalization of the hexa-cata-hexabenzocoronene is observed. The fixation of a CO2 molecule modifies the electronic properties. It enhances the capture of additional CO₂ molecules by changing the acidy and basicity of the rest of the boron and phosphorus atoms in the B3P3-NG system.

DOI: 10.1039/D2CP03999K  (OpenAccess)

Reduced nucleophilicity: an intrinsic property of the Lewis base atom interacting with H in hydrogen-bonds with Lewis acids HX (X = F, Cl, Br, I, CN, CCH, CP)

Equilibrium hydrogen-bond dissociation energies De for the process B⋯HX = B + HX are calculated at the CCSD(T)(F12c)/cc-pVDZ-F12 level for ∼190 complexes B⋯HX. As established earlier, De values for such complexes can be described by the equation De = cNBEA, in which NB and EHX are the nucleophilicity and electrophilicity of the Lewis base B and the Lewis acid HX, respectively, and the constant c = 1 kJ mol−1. Graphs of De as the ordinate and EHX as the abscissa are presented for 26 series of hydrogen-bonded complexes B⋯HX. The Lewis base is fixed and HX is HF, HCl, HBr, HI, HCN, HCCH and HCP in each series. Each plot yields a good straight line, the slope of which is the nucleophilicity NB of B. The Lewis bases are chosen for their simplicity and all have at least one non-bonding electron pair carried by the atom directly involved in the B⋯HX hydrogen bond. The direction of the minimum value σmin of the molecular electrostatic surface potential on the 0.001 e bohr−3 iso-surface in the chosen bases B usually coincides with the axis of a non-bonding electron pair. The gradient of a graph of De/σmin plotted against EHX defines a reduced nucleophilicity ИB = NB/σmin in the sense that ИB appears to be a property only of the atom of B that is directly involved in the B⋯HX hydrogen bond, independent of the remainder of B. For example, the values of the reduced nucleophilicity for the series of isocyanide complexes CH3NC⋯HX, HNC⋯HX and FNC⋯HX are 0.0343(16), 0.0337(18) and 0.0332(18), respectively, while those for the corresponding series of cyanide complexes are 0.0337(23), 0.0329(24) and 0.0333(23).

DOI: 10.1002/qua.26986  (OpenAccess)

Anion complexes of diborane derivatives inserted to benzene

The complexes formed between three molecules where the diboryl units (HBH2BH) replace the C2H2 moiety in benzene (B6H12, C2B4H10, and C4B2H8) and 10 anions (H−, F−, Cl−, Br−, OH−, CCH−, CH3−, CN−, N3−, and CNO−) have been studied using MP2 computational methods with the complete basis set (CBS) extrapolation scheme. The stability of the complexes for a given anion increases with the number of diborane units in the molecules, and these energy values are correlated with the presence of different anions and number of diborane units. The electron density of the complexes has been characterized and analyzed using the quantum theory of atoms in molecules (QTAIM) and electron density shift (EDS) models.

DOI: 10.1002/cphc.202200204 (OpenAccess)

Use of 5,10-Disubstituted Dibenzoazaborines and Dibenzophosphaborines as Cyclic Supports of Frustrated Lewis Pairs for the Capture of CO2

The reactivity of 5,10-disubstituted dibenzoazaborines and dibenzophosphaborines towards carbon dioxide is studied at the DFT level. The profile of this reaction comprises of three stationary points: the pre-reactive complex and adduct minima and the TS linking both minima. An equation is developed relating the properties (acidity, basicity and boron hybridization) of the isolated dibenzophosphaborine derivatives with the CO2 adduct energy.

DOI: 10.1039/D2CP01611G

Characterizing the n→π* interaction of pyridine with small ketones: a rotational study of pyridine⋯acetone and pyridine⋯2-butanone

Complexes formed by pyridine and small ketones such as acetone and 2-butanone have been generated in a supersonic jet and characterized by broadband Fourier transform microwave spectroscopy combined with high-level theoretical computations. The spectra of the complexes show a quadrupole coupling hyperfine structure due to the presence of a nitrogen atom and the splittings owing to the low barriers of the internal rotation of the methyl groups bonded to the carbonyl group. The corresponding barriers have been determined from the analysis of the spectra. We show in both complexes that pyridine closes a cycle with a ketone carbonyl group through an N⋯C=O n→π* tetrel interaction and a C–H⋯O contact. The n→π* tetrel bond involves the pyridine N atom lone pair and the ketone carbonyl group with a geometry approaching the Bürgi–Dunitz trajectory for the nucleophilic attack to a carbonyl group.

DOI: 10.1039/D2CP00021K   (OpenAccess)

Theoretical study of the NO3 radical reaction with CH2ClBr, CH2ICl, CH2BrI, CHCl2Br, and CHClBr2

The potential reaction of the nitrate radical (NO3), the main nighttime atmospheric oxidant, with five alkyl halides, halons (CH2ClBr, CH2ICl, CH2BrI, CHCl2Br, and CHClBr2) has been studied theoretically. The most favorable reaction corresponds to a hydrogen atom transfer. The stationary points on the potential energy surfaces of these reactions have been characterized. The reactions can be classified into two groups based on the number of hydrogen atoms in the halon molecules (1 or 2). The reactions with halons with only one hydrogen atom show more exothermic profiles than those with two hydrogen atoms. In addition, the kinetics of the reaction of NO3 + CH2BrI was studied in much higher detail using a multi-well Master Equation solver as a representative example of the nitrate radical reactivity against these halocarbons. These results indicate that the chemical lifetime of the alkyl halides would not be substantially affected by nitrate radical reactions, even in the case of NO3-polluted atmospheric conditions.

DOI: 10.1039/D2CP01565J  (OpenAccess)

Nucleophilicity of the boron atom in compounds R–B, (R = F, Cl, Br, I, CN, NC, CH3, SiH3, CF3, H): a new look at the inductive effects of the group R

Nucleophilicities NR–B of molecules R–B (R = F, Cl, Br, I, CN, NC, CH3, SiH3, CF3, H) are determined from the equilibrium dissociation energies De of 70 hydrogen-bonded complexes R–B⋯HX (X = F, Cl, Br, I, HCN. HCCH, HCP). The change in NR–B relative to NH–B of H–B allows a quantitative measure of the inductive effect IR of each group R because only the group R affects the electron density associated with the axial non-bonding electron pair carried by the boron in R–B. An alternative definition of IR, suggested by the strong correlation of the NR–B values with the minimum value σmin of the molecular electrostatic surface potential on the 0.001 e Bohr−3 iso-surface along the R–B axis leads to ex..cellent agreement between the two definitions.

DOI: 10.1063/5.0089716   (OpenAccess)

Disrupting bonding in azoles through beryllium bonds: Unexpected coordination patterns and acidity enhancement

Although triazoles and tetrazole are amphoteric and may behave as weak acids, the latter property can be hugely enhanced by beryllium bonds. To explain this phenomenon, the structure and bonding characteristics of the complexes between triazoles and tetrazoles with one and two molecules of BeF2 have been investigated through the use of high-level G4 ab initio calculations. The formation of the complexes between the N basic sites of the azoles and the Be center of the BeF2 molecule and the (BeF2)2 dimer leads to a significant bonding perturbation of both interacting subunits. The main consequence of these electron density rearrangements is the above-mentioned increase in the intrinsic acidity of the azole subunit, evolving from a typical nitrogen base to a very strong nitrogenous acid. This effect is particularly dramatic when the interaction involves the (BeF2)2 dimer, that is, a Lewis acid much stronger than the monomer. Although the azoles investigated have neighboring N-basic sites, their interaction with the (BeF2)2 dimer yields a monodentate complex. However, the deprotonated species becomes extra-stabilized because a second N–Be bond is formed, leading to a new N-N-Be(F2)-Be- five-membered ring, with the result that the azole-(BeF2)2 complexes investigated become stronger nitrogenous acids than oxyacids such as perchloric acid.

DOI: 10.1002/cphc.202200088

Stand up for Electrostatics: The Disiloxane Case

Molecular Electrostatic Potential is used to explain why Lewis basicity in disiloxane increases when the Si−O−Si angle is reduced. Arguments based on atomic charges seem to contradict the observed trends and could (wrongly) make one think that electrostatic considerations fail to provide a proper understanding of this system. However, the present study shows that this is not the case and highlights once again the importance of electrostatics and polarization in the description of noncovalent interactions.

DOI: 10.1039/d2cp00779g  (OpenAccess)

A reduced electrophilicity for simple Lewis acids A involved in non-covalent interactions with Lewis bases

Dissociation energies De for B⋯A = B + A can be written De = c′NBEA, where NB and EA are the nucleophilicities and electrophilicities of the Lewis base B and the Lewis acid A, respectively. A reduced electrophilicity is defined ΞA = EA/σmax, where σmax is the maximum electrostatic surface potential on iso-surface of A, the atom directly involved in the non-covalent interaction. This definition is tested for halogen-bonded complexes B⋯YX, with Lewis bases B = N2, CO, C2H2, C2H4, H2S, HCN H2O or NH3. De plotted against NB for several series B⋯YX yields straight lines of gradient EA. When De/σmax is the ordinate, the straight lines conflate to a single line, gradient ΞIX = EIX/σmax. Hydrogen-bonded complexes B⋯HX (X = F, Cl, Br, I), coinage-metal complexes B⋯MX (M = Cu, Ag, Au; X = F, Cl, Br, I), and alkali-metal bonded complexes B⋯MX (M = Li, Na: X = F, H, CH3) behave similarly. ΞA is an intrinsic property of the atom immediately involved in the non-covalent bond.

DOI: 10.3390/molecules27010017   (OpenAccess)

Perfluorination of Aromatic Compounds Reinforce Their van der Waals Interactions with Rare Gases: The Rotational Spectrum of Pentafluoropyridine-Ne

The rotational spectrum of the pentafluoropyridine-Ne complex, generated in a supersonic jet, has been investigated using chirped-pulse microwave Fourier transform spectroscopy in the 2–8 GHz range. The spectra of the 20Ne and 22Ne species have been observed, and the rotational constants have been used to determine the structure of the complex. This structure, and those of the previously experimentally studied complexes benzene-Ne and pyridine-Ne, are an excellent benchmark for the theoretical calculations on these adducts. These complexes and hexafluorobenzene-Ne have been investigated at the CCSD/6-311++G(2d,p) level. The calculations reproduce the experimental structures well and show how the van der Waals complexes are stronger for the perfluorinated compound.

DOI: 10.1002/slct.202103919

Anion Complexation Strongly Influences the Reactivity of Octafluorocyclooctatetraene

Six stationary points (three minima and three TS) in the thermal energy profile of octafluorocyclooctatetraene (F8-COT) have been explored. The TSs corresponding to the inversion process of F8-COT and its transformation in perfluoro bicyclo[2.2.2]octa-2,5,7-triene (F8-BOT) and subsequent to perfluoro tetracyclo[4.2.0.02.02,8,05,7]octene (F8-TCO) have been characterized. The results have been compared to the energy profile in the parent compound (cyclooctatetraene or COT). In addition, the effect of the formation of anion-π complexes in the reaction profile has been explored. The results have been rationalized using the electrostatic potential of the isolated stationary points and thermodynamic cycles involving the complex formation in the minima and TS and the barriers in the isolated molecules. Moreover, the electron density has been analyzed with the AIM methodology.

DOI: 10.1139/cjc-2021-0072

1,2-Dihydro-1,3,2-diazaborinine tautomer as an electron-pair donor in hydrogen-bonded complexes

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to investigate 1,2-dihydro-1,3,2-diazaborinine:HX complexes for HX = H+, HF, HCl, H2O, HCN, NH3, HCP, and HCCH. Most complexes are stabilized by linear, traditional hydrogen bonds except for those with H2O and NH3, which have bridging structures and nonlinear hydrogen bonds. H-atom transfer from N to B can occur in complexes with HF and HCl, with formation of a traditional F–H···N bond and a proton-shared Cl···H···N bond. The binding energies of the uncharged complexes range from 25 to 88 kJ mol−1. Spin-spin coupling constants have been used to characterize these hydrogen-bonded complexes.

DOI: 10.1002/ejic.202100737

Clustering of Electron Deficient B- and Be-Containing Analogues: In the Fight for Tetracoordination, Beryllium Takes the Lead

The stability of clusters involving Be- and B-containing analogues is dictated by the number of Be−Be and Be−B interactions with a clear preference for maximizing facing Be subunits. The most striking consequence is that in mixed trimers and tetramers the global minima always have Be-derivatives at the center.

DOI: 10.1002/chem.202102163 OpenAccess

How Aromatic Fluorination Exchanges the Interaction Role of Pyridine with Carbonyl Compounds: The Formaldehyde Adduct

The rotational spectrum of the weakly bound complex pentafluoropyridine⋅⋅⋅formaldehyde has been investigated using Fourier transform microwave spectroscopy. From the analysis of the rotational parameters of the parent species and of the 13C and 15N isotopologues, the structural arrangement of the adduct has been unambiguously established. The full ring fluorination of pyridine has a dramatic effect on its binding properties: It alters the electron density distribution at the π-cloud of pyridine creating a π-hole and changing its electron donor-acceptor capabilities. In the complex, formaldehyde lies above the aromatic ring with one of the oxygen lone pairs, as conventionally envisaged, pointing toward its centre. This lone pair⋅⋅⋅π-hole interaction, reinforced by a weak C−H⋅⋅⋅N interaction, indicates an exchange of the electron-acceptor roles of both molecules when compared to the pyridine⋅⋅⋅formaldehyde adduct. Tunnelling doublets due to the internal rotation of formaldehyde have also been observed and analysed leading to a discussion on the competition between lone pair⋅⋅⋅π-hole and π⋅⋅⋅π stacking interactions.

DOI: 10.1021/acs.jpca.1c04787  (OpenAccess)

Sequestration of Carbon Dioxide with Frustrated Lewis Pairs Based on N-Heterocycles with Silane/Germane Groups

Frustrated Lewis pairs (FLPs) based on nitrogen heterocycles (pyridine, pyrazole, and imidazole) with a silane or germane group in the α-position of a nitrogen atom have been considered as potential molecules to sequestrate carbon dioxide. Three stationary points have been characterized in the reaction profile: a pre-reactive complex, an adduct minimum, and the transition state connecting them. The effect of external (solvent) or internal (hydroxyl group) electric fields in the reaction profile has been considered. In both cases, it is possible to improve the kinetics and thermodynamics of the complexation of CO2 by the FLP and favor the formation of adducts.

DOI: 10.1002/cplu.202100235  (OpenAccess)

Alkylammonium Cation Affinities of Nitrogenated Organobases: The Roles of Hydrogen Bonding and Proton Transfer

Alkylammonium cation affinities of 64 nitrogen-containing organobases, as well as the respective proton transfer processes from the alkylammonium cations to the base, have been computed in the gas phase by using DFT methods. The guanidine bases show the highest proton transfer values (191.9–233 kJ mol−1) whereas the cis-2,2’-biimidazole presents the largest affinity towards the alkylammonium cations (>200 kJ mol−1) values. The resulting data have been compared with the experimentally reported proton affinities of the studied nitrogen-containing organobases revealing that the propensity of an organobase for the proton transfer process increases linearly with its proton affinity. This work can provide a tool for designing senors for bioactive compounds containing amino groups that are protonated at physiological pH.

DOI: 10.1080/00268976.2021.1933637  OpenAccess

Microsolvation of the Be-F bond in complexes of BeF₂, BeF₃^–1, and BeF₄^–2 with nH₂O, for n = 1–6

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to determine the structures and binding energies of complexes of BeF2, BeF3–1, and BeF4–2 with n H2O molecules, with n = 1–6. For each series of complexes with BeF2, BeF3–1, and BeF4–2, the binding energies increase as the number of water molecules increases, but the binding energies per intermolecular bond for complexes formed between BeF2 and BeF4–2 and nH2O decrease as the number of water molecules increases. The binding energies per bond of the BeF3–1 complexes show little dependence on the number of H2O molecules present. Intermolecular O-H … F hydrogen bonds (HB) stabilise all complexes of H2O with the beryllium bases except for BeF2:1H2O and BeF2:2H2O. Complexes of BeF2 are also stabilised by O … Be beryllium bonds (BeB) and O-H … O hydrogen bonds. EOM-CCSD calculations have also been performed to determine the spin–spin coupling constants. The one-bond coupling constants 1J(Be-F) increase as the Be-F distance decreases, and exhibit an excellent second-order correlation with that distance. 2hJ(O-F) coupling constants across O-H … F hydrogen bonds also exhibit a second-order dependence on distance. Coupling constants 1beJ(O-Be) and 2hJ(O-O) are found in complexes with BeF2 and exhibit a linear dependence on the Be-O and O-O distances, respectively.

DOI: 10.1016/j.cplett.2021.138809  (OpenAccess)

Stationary states of systems with intermolecular interactions dominated by electrostatics: Structure of trimethylammonium and tetramethylammonium chlorides and bromides in the gas phase, monomers and dimers

This work reports the theoretical study carried out with the M06-2x functional and the aug-cc-pVTZ basis set of four ammonium and two phosphonium salts, Me₃NH⁺Cl^–, Me₃NH⁺Br^–, Me₄N⁺Cl^–, Me₄N⁺Br^–, Me₄P⁺Cl^– and Me₄P⁺Br^–. The structure of the monomeric and dimeric complexes (between 1 and 5 conformations each) has been analyzed in what concern geometries, energies and NH+ stretching frequencies. The ammonium geometries were successfully compared with the X-ray structures found in the CSD.

DOI: 10.1021/acs.jpca.1c00830  (OpenAccess)

Evaluation of Electron Density Shifts in Noncovalent Interactions

In the present paper, we report the quantitative evaluation of the electron density shift (EDS) maps within different complexes. Values associated with the total EDS maps exhibited good correlation with different quantities such as interaction energies, Eint, intermolecular distances, bond critical points, and LMOEDA energy decomposition terms. Besides, EDS maps at different cutoffs were also evaluated and related with the interaction energies values. Finally, EDS maps and their corresponding values are found to correlate with Eint within systems with cooperative effects. To our knowledge, this is the first time that the EDS has been quanitatively evaluated.

DOI: 10.3390/molecules26113401 (OpenAccess)

Large Stabilization Effects by Intramolecular Beryllium Bonds in Ortho-Benzene Derivatives

Intramolecular interactions are shown to be key for favoring a given structure in systems with a variety of conformers. In ortho-substituted benzene derivatives including a beryllium moiety, beryllium bonds provide very large stabilizations with respect to non-bound conformers and enthalpy differences above one hundred kJ·mol⁻¹ are found in the most favorable cases, especially if the newly formed rings are five or six-membered heterocycles. These values are in general significantly larger than hydrogen bonds in 1,2-dihidroxybenzene. Conformers stabilized by a beryllium bond exhibit the typical features of this non-covalent interaction, such as the presence of a bond critical point according to the topology of the electron density, positive Laplacian values, significant geometrical distortions and strong interaction energies between the donor and acceptor quantified by using the Natural Bond Orbital approach. An isodesmic reaction scheme is used as a tool to measure the strength of the beryllium bond in these systems in terms of isodesmic energies (analogous to binding energies), interaction energies and deformation energies. This approach shows that a huge amount of energy is spent on deforming the donor–acceptor pairs to form the new rings.

DOI: 10.3390/molecules26113086  (OpenAccess)

Perturbing the O–H···O Hydrogen Bond in 1‐Oxo‐3‐hydroxy‐2‐propene

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to identify and characterize equilibrium structures and transition structures on the 1-oxo-3-hydroxy-2-propene: Lewis acid potential energy surfaces, with the acids LiH, LiF, BeH₂, and BeF₂. Two equilibrium structures, one with the acid interacting with the C=O group and the other with the interaction occurring at the O–H group, exist on all surfaces. These structures are separated by transition structures that present the barriers to the interconversion of the two equilibrium structures. The structures with the acid interacting at the C=O group have the greater binding energies. Since the barriers to convert the structures with interaction occurring at the O–H group are small, only the isomers with interaction occurring at the C=O group could be experimentally observed, even at low temperatures. Charge-transfer energies were computed for equilibrium structures, and EOM-CCSD spin–spin coupling constants ^2hJ(O–O), ^1hJ(H–O), and ¹J(O–H) were computed for equilibrium and transition structures. These coupling constants exhibit a second-order dependence on the corresponding distances, with very high correlation coefficients.

DOI: 10.1002/cplu.202100088  (OpenAccess)

The Electrophilicities of XCF₃ and XCl (X=H, Cl, Br, I) and the Propensity of These Molecules To Form Hydrogen and Halogen Bonds with Lewis Bases: An Ab Initio Study

Equilibrium dissociation energies, De, of four series of halogen‐ and hydrogen‐bonded complexes B⋅⋅⋅XCF₃ (X=H, Cl, Br and I) are calculated ab initio at the CCSD(T)(F12c)/cc‐pVDZ‐F12 level. The Lewis bases B involved are N₂, CO, PH₃, C₂H₂, C₂H₄, H₂S, HCN, H₂O and NH₃. Plots of De versus NB, where the NB are the nucleophilicities assigned to the Lewis bases previously, are good straight lines through the origin, as are those for the corresponding set of complexes B⋅⋅⋅XCl. The gradients of the De versus NB plots define the electrophilicities E_XCF3 and E_XCl of the various Lewis acids. The determined values are: E_XCF3=2.58(22), 1.40(9), 2.15(2) and 3.04(9) for X=H, Cl, Br and I, respectively, and E_XCl=4.48(22), 2.31(9), 4.37(27) and 6.06(37) for the same order of X. Thus, it is found that, for a given X, the ratio E_XCl/ E_XCF3 is 2 within the assessed errors, and therefore appears to be independent of the atom X and of the type of non‐covalent interaction (hydrogen bond or different varieties of halogen bond) in which it is involved. Consideration of the molecular electrostatic surface potentials shows that De and the maximum positive electrostatic potential σmax (the most electrophilic region of XCF₃ and XCl, which lies on the symmetry axes of these molecules, near to the atom X) are strongly correlated.

DOI: 10.1080/00268976.2021.1905191  (OpenAccess)

IR and NMR properties of N-base:PH2F:BeX2 ternary and corresponding binary complexes stabilised by pnicogen and beryllium bonds

Ab initio MP2/aug’-cc-pVTZ calculations have been performed to determine selected stretching frequencies and chemical shieldings for ternary complexes N-base:PH2F:BeX2 and the corresponding binary complexes, with NH3, H2C=NH, and HCN as the nitrogen bases and H, F, and Cl as the substituents X. Be-F and P-F stretching frequencies depend on the Be-F and P-F distances, respectively, while P and F chemical shieldings depend on the N-P and P-F distances, respectively. The graph of the P chemical shieldings versus the P-F distance bears a remarkable resemblance to the graph of the P-F stretching frequencies versus that same distance. EOM-CCSD spin–spin coupling constants have also been evaluated for binary and ternary complexes. 1pJ(N-P) is negative at the longer N-P distances found in ternary complexes with HCN in which the N···P bond is a traditional pnicogen bond with some phosphorous-shared character, gains phosphorus-shared character as the N-P distance continues to decrease, and then becomes a phosphorous-transferred bond with ion-pair character at the shorter distances in the complexes with NH3 and H2C=NH. 1J(P-F) values are large and negative in complexes with HCN, but increase and become positive in complexes with H2C=NH and NH3 as the P-F distance increases.

DOI: 10.3390/cryst11040391  OpenAccess

Carboranes as Lewis Acids: Tetrel Bonding in CB11H11 Carbonium Ylide

High-level quantum-chemical computations (G4MP2) are carried out in the study of complexes featuring tetrel bonding between the carbon atom in the carbenoid CB₁₁H₁₁—obtained by hydride removal in the C-H bond of the known closo-monocarbadodecaborate anion CB₁₁H₁₂⁽⁻⁾ and acting as Lewis acid (LA)—and Lewis bases (LB) of different type; the electron donor groups in the tetrel bond feature carbon, nitrogen, oxygen, fluorine, silicon, phosphorus, sulfur, and chlorine atomic centres in neutral molecules as well as anions H⁽⁻⁾, OH⁽⁻⁾, and F⁽⁻⁾. The empty radial 2p_r vacant orbital on the carbon centre in CB₁₁H₁₁, which corresponds to the LUMO, acts as a Lewis acid or electron attractor, as shown by the molecular electrostatic potential (MEP) and electron localization function (ELF). The thermochemistry and topological analysis of the complexes {CB₁₁H₁₁:LB} are comprehensively analysed and classified according to shared or closed-shell interactions. ELF analysis shows that the tetrel C⋯X bond ranges from very polarised bonds, as in H₁₁B₁₁C:F^(-) to very weak interactions as in H₁₁B₁₁C⋯FH and H₁₁B₁₁C⋯O=C=O.

DOI: 10.1039/d0cp06009g

Spontaneous bond dissociation cascades induced by Be_n clusters (n = 2,4)

High-level single and multireference ab initio calculations show that the Be4 cluster behaves as a very efficient Lewis acid when interacting with conventional Lewis bases such as ammonia, water or hydrogen fluoride, to the point that the corresponding acid–base interaction triggers a sequential dissociation of all the bonds of the Lewis base. Notably, this behavior is already found for the simplest beryllium cluster, the Be2 dimer. However, whereas for Be2 the first dissociation process involves a low activation barrier which is above the reactants, for Be4 all the bond dissociation processes involve barriers below the entrance channel leading to a cascade of successive exothermic processes, which end up spontaneously in a global minimum in which the bonding patterns of both the base and the Lewis acid are completely destroyed. Indeed, the global minimum, in all cases, is stabilized by three-center Be–H–Be bonds and covalent interactions between the Be atoms and the basic center of the base, which replace the initial metallic bond stabilizing the Be4 cluster. As a consequence, in the global minimum the basic atoms (N, O and F) behave as hyper-coordinated centers. Also importantly, the Be4 cluster and its complexes present RHF–UHF instabilities (not reported before for Be4), which require the use of multireference methods to correctly describe them.

DOI: 10.3390/inorganics9020013  OpenAccess

Non-Covalent Interactions of the Lewis Acids Cu–X, Ag–X, and Au–X (X = F and Cl) with Nine Simple Lewis Bases B: A Systematic Investigation of Coinage–Metal Bonds by Ab Initio Calculations

The equilibrium geometry and two measures (the equilibrium dissociation energy in the complete basis set limit, D_e(CBS) and the intermolecular stretching force constant k_σ) of the strength of the non-covalent interaction of each of six Lewis acids M–X (M = Cu, Ag, Au) with each of nine simple Lewis bases B (B = N₂, CO, HCCH, CH₂CH₂, H₂S, PH₃, HCN, H₂O, and NH₃) have been calculated at the CCSD(T)/aug-cc-pVTZ level of theory in a systematic investigation of the coinage–metal bond. Unlike the corresponding series of hydrogen-bonded B⋯HX and halogen-bonded B⋯XY complexes (and other series involving non-covalent interactions), D_e is not directly proportional to k_σ. Nevertheless, as for the other series, it has been possible to express D_e in terms of the equation D_e = cN_B.E_MX, where N_B and E_MX are the nucleophilicities of the Lewis bases B and the electrophilicities of the Lewis acids M–X, respectively. The order of the E_MX is determined to be E_AuF > E_AuCl > E_CuF > E_CuCl > E_AgF≈E_AgCl. A reduced electrophilicity defined as (E_MX/σ_max) is introduced, where σ_max is the maximum positive value of the molecular electrostatic surface potential on the 0.001 e/bohr³ iso-surface. This quantity is, in good approximation, independent of whether F or Cl is attached to M.

DOI: 10.1002/cphc.202000412

From Very Strong to Inexistent Be-Be Bonds in the Interactions of Be2 with π-Systems

Isolated Be2 is a typical example of a weakly bound system, but interaction with other systems may give rise to surprising bonding features. The interactions between Be2 and a set of selected neutral CnHn (n=2–8) π‐systems have been analyzed through the use of G4 and G4MP2 ab initio methods, along with multireference CASPT2//CASPT2 calculations. Our results systematically show that the CnHn−Be2−CnHn clusters formed are always very stable. However, the nature of this interaction is completely different when the π‐system involved is a closed shell species (n=2, 4, 6, 8), or a radical (n=3, 5, 7). In the first case, the interaction does not occur with the π‐system as a whole, but with specific C centers yielding rather polar but strong C−Be bonds. Nonetheless, although the Be−Be distances in these complexes are similar to the ones in compounds with ultra‐strong Be−Be bonds, a close examination of their electron density distribution reveals that no Be−Be bonds exist. The situation is totally different when the interaction involves two π‐radicals, CnHn−Be2−CnHn (n=3, 5, 7). In these cases, a strong Be−Be bond is formed. Indeed, even though Be is electron deficient, the Be2 moiety behaves as an efficient electron donor towards the two π‐radicals, so that the different CnHn−Be2−CnHn (n=3, 5, 7) clusters are the result of the interaction between Be22+ and two L− anions. The characteristics of these two scenarios do not change when dealing with bicyclic π‐compounds, such as naphthalene and pentalene, because the interaction with the Be2 moiety is localized on one of the unsaturated cycles, the other being almost a spectator.

DOI: 10.3390/molecules25245876  OpenAccess

The Importance of Strain (Preorganization) in Beryllium Bonds

In order to explore the angular strain role on the ability of Be to form strong beryllium bonds, a theoretical study of the complexes of four beryllium derivatives of orthocloso-carboranes with eight molecules (CO, N₂, NCH, CNH, OH₂, SH₂, NH₃, and PH₃) acting as Lewis bases has been carried out at the G4 computational level. The results for these complexes, which contain besides Be other electron-deficient elements, such as B, have been compared with the analogous ones formed by three beryllium salts (BeCl₂, CO₃Be and SO₄Be) with the same set of Lewis bases. The results show the presence of large and positive values of the electrostatic potential associated to the beryllium atoms in the isolated four beryllium derivatives of ortho-carboranes, evidencing an intrinsically strong acidic nature. In addition, the LUMO orbital in these systems is also associated to the beryllium atom. These features led to short intermolecular distances and large dissociation energies in the complexes of the beryllium derivatives of ortho-carboranes with the Lewis bases. Notably, as a consequence of the special framework provided by the ortho-carboranes, some of these dissociation energies are larger than the corresponding beryllium bonds in the already strongly bound SO₄Be complexes, in particular for N₂ and CO bases. The localized molecular orbital energy decomposition analysis (LMOEDA) shows that among the attractive terms associated with the dissociation energy, the electrostatic term is the most important one, except for the complexes with the two previously mentioned weakest bases (N₂ and CO), where the polarization term dominates. Hence, these results contribute to further confirm the importance of bending on the beryllium environment leading to strong interactions through the formation of beryllium bonds.

DOI: 10.1002/cphc.202000704

Rivalry between Regium and Hydrogen Bonds Established within Diatomic Coinage Molecules and Lewis Acids/Bases

A theoretical study of the complexes formed by Ag₂ and Cu₂ with different molecules, XH (FH, ClH, OH₂, SH₂, HCN, HNC, HCCH, NH₃ and PH₃) that can act as hydrogen‐bond donors (Lewis acids) or regium‐bond acceptors (Lewis bases) was carried out at the CCSD(T)/CBS computational level. The heteronuclear diatomic coinage molecules (AuAg, AuCu, and AgCu) have also been considered. With the exception of some of the hydrogen‐bonded complexes with FH, the regium‐bonded binary complexes are more stable. The AuAg and AuCu molecules show large dipole moments that weaken the regium bond (RB) with Au and favour those through the Ag and Cu atoms, respectively.

DOI: 10.1016/j.cplett.2020.137916

Probing the structures, binding energies, and spin-spin coupling constants of halogen-bonded Azine:ClF complexes

Ab initio MP2/aug’-cc-pVTZ calculations were performed to determine the structures, binding energies, and charge-transfer energies of azine:ClF complexes. Binding energies and charge-transfer energies exhibit excellent correlations with the N-Cl distance, decreasing as the number of N atoms increases. EOM-CCSD spin-spin coupling constants 1xJ(N-Cl) do not correlate well with the N-Cl distance, although patterns emerge for each subgroup of complexes that have the same number of nitrogen atoms in the ring. The average value of the coupling constants for each subgroup exhibits an excellent correlation with the average distance, and illustrates the changing nature of the halogen bond.

DOI: 10.1039/D0CE01272F

A C(pi-hole)⋯Cl–Zn tetrel interaction driving a metal–organic supramolecular assembly

A brominated pyrimidinyl triazolopyridine ligand (bptpy) forms a C(π-hole)⋯Cl–Zn tetrel interaction that plays a determining role in the formation of supramolecular layers through chain assembly in 18 membered metallacycle [(ZnCl2)2(μ-bptpy)2] crystals. Supramolecular chains are formed through C–H⋯X interactions. The observed interactions are supported by DFT calculations using model dimers.

DOI: 10.3390/ijms21218036  (OpenAccess)

Interaction between Trinuclear Regium Complexes of Pyrazolate and Anions, a Computational Study

The geometry, energy and electron density properties of the 1:1, 1:2 and 1:3 complexes between cyclic (Py-M)₃ (M = Au, Ag and Cu) and halide ions (F⁻, Cl⁻ and Br⁻) were studied using Møller Plesset (MP2) computational methods. Three different configurations were explored. In two of them, the anions interact with the metal atoms in planar and apical dispositions, while in the last configuration, the anions interact with the CH(4) group of the pyrazole. The energetic results for the 1:2 and 1:3 complexes are a combination of the specific strength of the interaction plus a repulsive component due to the charge:charge coulombic term. However, stable minima structures with dissociation barriers for the anions indicate that those complexes are stable and (Py-M)₃ can hold up to three anions simultaneously. A search in the CSD confirmed the presence of (Pyrazole-Cu)₃ systems with two anions interacting in apical disposition.

DOI: 10.1002/anie.202007814 

Spodium Bonds: Noncovalent Interactions Involving Group 12 Elements

The term spodium (Sp) bond is proposed to refer to a net attractive interaction between any element of Group 12 and electron‐rich atoms (Lewis bases or anions). These noncovalent interactions are markedly different from coordination bonds (antibonding Sp–ligand orbital involved). Evidence is provided for the existence of this interaction by calculations at the RI‐MP2/aug‐cc‐pVTZ level of theory, atoms‐in‐molecules, and natural bond orbital analyses and by examining solid‐state structures in the Cambridge Structure Database.

DOI: 10.1039/d0cp02697b

An ab initio investigation of alkali–metal noncovalent bonds B···LiR and B···NaR (R = F, H or CH₃) formed with simple Lewis bases B: the relative inductive effects of F, H and CH₃

The alkali–metal bonds formed by simple molecules LiR and NaR (R = F, H or CH3) with each of the six Lewis bases B = OC, HCN, H2O, H3N, H2S and H3P were investigated by ab initio calculations at the CCSD(T)/AVTZ and CCSD(T)/awCVTZ levels of theory with the aim of characterising this type of non-covalent interaction. In some complexes, two minima were discovered, especially for those involving the NaR. The higher-energy minimum (referred to as Type I) for a given B was found to have geometry that is isomorphous with that of the corresponding hydrogen-bonded analogue B⋯HF. The lower-energy minimum (when two were present) showed evidence of a significant secondary interaction of R with the main electrophilic region of B (Type II complexes). Energies DCBSe for dissociation of the complexes into separate components were found to be directly proportional to the intermolecular stretching force constant kσ The value of DCBSe could be partitioned into a nucleophilicity of B and an electrophilicity of LiR or NaR, with the order ELiH ≳ ELiF = ELiCH3 for the LiR and ENaF > ENaH ≈ ENaCH3 for the NaR. For a given B, the order of the electrophilicities is ELiR > ENaR, which presumably reflects the fact that Li+ is smaller than Na+ and can approach the Lewis base more closely. A SAPT analysis revealed that the complexes B⋯LiR and B⋯NaR have larger electrostatic contributions to De than do the hydrogen- and halogen-bonded counterparts B⋯HCl and B⋯ClF.

DOI: 10.1039/d0cp02697b

An ab initio investigation of alkali–metal noncovalent bonds B···LiR and B···NaR (R = F, H or CH₃) formed with simple Lewis bases B: the relative inductive effects of F, H and CH₃

The alkali–metal bonds formed by simple molecules LiR and NaR (R = F, H or CH3) with each of the six Lewis bases B = OC, HCN, H2O, H3N, H2S and H3P were investigated by ab initio calculations at the CCSD(T)/AVTZ and CCSD(T)/awCVTZ levels of theory with the aim of characterising this type of non-covalent interaction. In some complexes, two minima were discovered, especially for those involving the NaR. The higher-energy minimum (referred to as Type I) for a given B was found to have geometry that is isomorphous with that of the corresponding hydrogen-bonded analogue B⋯HF. The lower-energy minimum (when two were present) showed evidence of a significant secondary interaction of R with the main electrophilic region of B (Type II complexes). Energies DCBSe for dissociation of the complexes into separate components were found to be directly proportional to the intermolecular stretching force constant kσ The value of DCBSe could be partitioned into a nucleophilicity of B and an electrophilicity of LiR or NaR, with the order ELiH ≳ ELiF = ELiCH3 for the LiR and ENaF > ENaH ≈ ENaCH3 for the NaR. For a given B, the order of the electrophilicities is ELiR > ENaR, which presumably reflects the fact that Li+ is smaller than Na+ and can approach the Lewis base more closely. A SAPT analysis revealed that the complexes B⋯LiR and B⋯NaR have larger electrostatic contributions to De than do the hydrogen- and halogen-bonded counterparts B⋯HCl and B⋯ClF.

DOI: 10.1039/d0cp02009e  (OpenAccess)

Hydrogen bonds and halogen bonds in complexes of carbones L→C←L as electron donors to HF and ClF, for L = CO, N₂, HNC, PH₃, and SH₂

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to determine the structures and binding energies of the carbone complexes in which the carbone L→C←L acts as an electron pair donor to one and two HF or ClF molecules, for L = CO, N2, HNC, PH3, and SH2. The binding energies increase with respect to the ligand in the order CO < NN < CNH ≪ PH3 < SH2, and increase with respect to the acid in the order HF < 2 HF < ClF < 2 ClF. The complexes with the ligands CO, N2 and PH3 have C2v symmetry while those with CNH and SH2 have Cs symmetry, except for H2S→C←SH2:2HF which has C2 symmetry and a unique structure among all of the carbone complexes. F–H and Cl–F stretching frequencies in the complexes decrease as the F–H and Cl–F distances, respectively, increase. EOM-CCSD spin–spin coupling constants 2hJ(F–C) increase with decreasing F–C distance. Although the F–H⋯C hydrogen bonds gain some proton-shared character in the most tightly bound complexes, the hydrogen bonds remain traditional hydrogen bonds. 1xJ(Cl–C) values indicate that the Cl⋯C halogen bonds have chlorine shared character even at the longest distances. 1xJ(Cl–C) then increases as the Cl–C distance decreases, and reaches a maximum for chlorine-shared halogen bonds. As the Cl–C distance further decreases, the halogen bond becomes a chlorine-transferred halogen bond.

DOI: 10.1021/acs.jpca.0c03689

Mutual Influence of Pnicogen Bonds and Beryllium Bonds: Energies and Structures in the Spotlight

Pnicogen bonds, which are weak noncovalent interactions (NCIs), can be significantly modified by the presence of beryllium bonds, one of the strongest NCIs known. We demonstrate the importance of this influence by studying ternary complexes in which both NCIs are present, that is, the ternary complexes formed by a nitrogen base (NH3, NHCH2, and NCH), a phosphine (fluorophosphane, PH2F) and a beryllium derivative (BeH2, BeF2, BeCl2, BeCO3, and BeSO4). Energies, structures, and nature of the chemical bonding in these complexes are studied by means of ab initio computational methods. The pnicogen bond between the nitrogen base and the phosphine and the beryllium bond between the fluorine atom of fluorophosphane and the beryllium derivative show large cooperativity effects both on energies and geometries, with dissociation energies up to 296 kJ mol–1 and cooperativity up to 104 kJ mol–1 in the most strongly bound complex, CH2HN:PH2F:BeSO4. In the complexes between the strongest nitrogen bases and the strongest beryllium donors, phosphorus-shared and phosphorus-transfer bonds are found.

DOI: 10.1002/cphc.202000172

Metastable Dianions and Dications

A theoretical study of metastable dianions and dications has been carried out at the CCSD(T)//MP2 level. MX₃²⁻ and LX₄²⁻ (M=Li and Na, L=Be and Mg, X=F and Cl) have been considered as dianions, M₃X²⁺ (M=Li and Na, X=F and Cl), YH₃²⁺ and ZH₄²⁺ (Y=F and Cl and Z=O, S) as dications. Minima structures are found in all cases, but they are less stable than the corresponding dissociated pair of mono‐ions. The dissociation profile of the molecules in two mono‐ions has been explored showing in all cases a maximum that prevent their spontaneous dissociation. The strength and nature of the chemical bond in the dianions and dications have been analyzed with the QTAIM, NBO and LMOEDA method and compared to the corresponding monoanions and monocations.

DOI: 10.1039/d0nj01803a

Bonding between electron-deficient atoms: strong Lewis-acid character preserved in X–Y–X (X = B, Al; Y = Be, Mg) bridges

The structure, stability and bonding of beryllium bis(diazaborolyl) derivatives and their Mg and Al-containing analogues have been investigated through the use of G4-high level ab initio methods. This survey allowed us to conclude that not only the Be/B compounds, recently reported in the literature, but also their analogues in which Be is replaced by Mg and B is replaced by Al should exhibit similar bonding characteristics and stability and therefore should be, in principle, also experimentally accessible. In general, in all these derivatives B and Al, in spite of being intrinsically electron-deficient systems, behave as nucleophiles towards Be and Mg. One of the consequences of having two electron-deficient atoms involved in a bond is that the net charge associated to both of them is usually positive, in contrast with typical Be or Mg containing compounds, such as BeF2 or MgF2. However, this apparent dissimilarity is not so, because of the role of the second neighbors. Indeed, in the systems investigated, even though the Be–B, Be–Al, Mg–B or Mg–Al bonds are not too polar as compared with Be–F or Mg–F, the cluster can be viewed as the interaction between a positively charged Be(Mg) atom with two negatively charged ligands. Consistently, the homolytic dissociation enthalpies of the Be–ligand bonds, though smaller than that of the Be–F one, are still significantly large. Also importantly, the Lewis acid character of Be is still preserved, as all the Be compounds investigated lead to rather strong beryllium bonds when interacting with one or two typical Lewis bases, such as methylamine. The same behavior is observed for the corresponding Mg-containing analogues.

DOI: 10.1007/s11224-020-01556-2

Complexes between bicyclic boron derivatives and dihydrogen: the importance of strain

The 1:1 complexes of neutral derivatives of borabicyclo[2.2.2] and borabicyclo[3.3.3] with dihydrogen have been studied by means of MP2 computational methods. In all cases, an interaction between the boron atom and the sigma bond of dihydrogen is observed. The chemical environment of the boron atom plays an important role in the observed intermolecular distance and binding energies. Very short intermolecular distances (B-H distances shorter than 1.5 Å) are obtained for several cases and binding energies up to − 97 kJ mol−1. The importance of strain has been confirmed with analogous non-cyclic systems that have been distorted to increase the non-planarity of the boron atom.

DOI: 10.3390/molecules25122846   OpenAccess

Unusual Complexes of P(CH)₃ with FH, ClH, and ClF

Ab initio MP2/aug’-cc-pVTZ calculations have been performed to determine the structures and binding energies of complexes formed by phosphatetrahedrane, P(CH)₃, and HF, HCl, and ClF. Four types of complexes exist on the potential energy surfaces. Isomers A form at the P atom near the end of a P-C bond, B at a C-C bond, C at the centroid of the C-C-C ring along the C₃ symmetry axis, and D at the P atom along the C₃ symmetry axis. Complexes A and B are stabilized by hydrogen bonds when FH and ClH are the acids, and by halogen bonds when ClF is the acid. In isomers C, the dipole moments of the two monomers are favorably aligned but in D the alignment is unfavorable. For each of the monomers, the binding energies of the complexes decrease in the order A > B > C > D. The most stabilizing Symmetry Adapted Perturbation Theory (SAPT) binding energy component for the A and B isomers is the electrostatic interaction, while the dispersion interaction is the most stabilizing term for C and D. The barriers to converting one isomer to another are significantly higher for the A isomers compared to B. Equation of motion coupled cluster singles and doubles (EOM-CCSD) intermolecular coupling constants J(X-C) are small for both B and C isomers. J(X-P) values are larger and positive in the A isomers, negative in the B isomers, and have their largest positive values in the D isomers. Intramolecular coupling constants ¹J(P-C) experience little change upon complex formation, except in the halogen-bonded complex FCl:P(CH₃) A.

DOI: 10.3390/molecules25112674   OpenAccess

An Interacting Quantum Atoms (IQA) and Relative Energy Gradient (REG) Study of the Halogen Bond with Explicit Analysis of Electron Correlation

Energy profiles of seven halogen-bonded complexes were analysed with the topological energy partitioning called Interacting Quantum Atoms (IQA) at MP4(SDQ)/6–31+G(2d,2p) level of theory. Explicit interatomic electron correlation energies are included in the analysis. Four complexes combine X₂ (X = Cl or F) with HCN or NH₃, while the remaining three combine ClF with HCN, NH₃ or N₂. Each complex was systematically deformed by translating the constituent molecules along its central axis linking X and N, and reoptimising its remaining geometry. The Relative Energy Gradient (REG) method (Theor. Chem. Acc. 2017, 136, 86) then computes which IQA energies most correlate with the total energy during the process of complex formation and further compression beyond the respective equilibrium geometries. It turns out that the covalent energy (i.e., exchange) of the halogen bond, X…N, itself drives the complex formation. When the complexes are compressed from their equilibrium to shorter X…N distance then the intra-atomic energy of N is in charge. When the REG analysis is restricted to electron correlation then the interatomic correlation energy between X and N again drives the complex formation, and the complex compression is best described by the destabilisation of the through-space correlation energy between N and the “outer” halogen.

DOI: 10.1039/d0cp01321h

Stabilisation of dianion dimers trapped inside cyanostar macrocycles

Interanionic H-bonds (IAHBs) are unfavourable interactions in the gas phase becoming favoured when anions are in solution. Dianion dimers are also susceptible to being trapped inside the cavities of cyanostar (CS) macrocycles, and thus, the formation of 2 : 2 anion : cyanostar aggregates is mainly supported by three kinds of interactions: IAHBs between the dianions, π–π stacking between the confronted cyanostars, and the presence of an intricate network of multiple C(sp2)H⋯O H-bonds between cyanostar ligands and the anionic moieties. An analysis of the interaction energies supported by NBO reveals a slight cooperative effect of the CSs on the IAHB stabilisation.

DOI: 10.1002/cphc.201901200

Anion-Anion Complexes Established between Aspartate Dimers

Stable dimers aspartate‐aspartate have been studied in aqueous and gas phase through theoretical simulations. The polarizable continuum model (PCM) has been applied to simulate the effect of the hydration on monomers and complexes. The quantum theory of atoms in molecules (QTAIM) and the interacting quantum atoms (IQA) scheme has been used to inquire into if, in the aqueous phase, individual hydrogen bonds have attractive electrostatic components. In all cases a spontaneous formation of the complexes in the aqueous phase are observed, while in the gas phase a considerable energy barrier must be overcome (between 100.8 to 263.2 kJ mol−1). The intermolecular distance at which this barrier is indicates when the hydrogen‐bond interactions begin to take importance between the dimers and the corresponding molecular recognition among them. The IQA analysis shows that in aqueous phase, the hydrogen bonds N−H⋅⋅⋅O are mainly electrostatic in nature with a certain covalent character which increases linearly with the decrease of internuclear distances H⋅⋅⋅O. The H⋅⋅⋅H interactions observed are stabilizing and they are mainly quantum in nature.

DOI: 10.3390/ma13092163, OpenAccess

Hydrogen vs. Halogen Bonds in 1-Halo-Closo-Carboranes

A theoretical study of the hydrogen bond (HB) and halogen bond (XB) complexes between 1-halo-closo-carboranes and hydrogen cyanide (NCH) as HB and XB probe has been carried out at the MP2 computational level. The energy results show that the HB complexes are more stable than the XBs for the same system, with the exception of the isoenergetic iodine derivatives. The analysis of the electron density with the quantum theory of atoms in molecules (QTAIM) shows the presence of a unique intermolecular bond critical point with the typical features of weak noncovalent interactions (small values of the electron density and positive Laplacian and total energy density). The natural energy decomposition analysis (NEDA) of the complexes shows that the HB and XB complexes are dominated by the charge-transfer and polarization terms, respectively. The work has been complemented with a search in the CSD database of analogous complexes and the comparison of the results, with those of the 1-halobenzene:NCH complexes showing smaller binding energies and larger intermolecular distances as compared to the 1-halo-closo-carboranes:NCH complexes.

DOI: 10.1002/cphc.202000099

Complexes H₂CO:PXH₂ and HCO₂H:PXH₂ for X=NC, F, Cl, CN, OH, CCH, CH₃, and H: Pnicogen Bonds and Hydrogen Bonds

Ab initio MP2/aug’‐cc‐pVTZ calculations have been carried out to investigate H2CO : PXH2 pnicogen‐bonded complexes and HCO2H : PXH2 complexes that are stabilized by pnicogen bonds and hydrogen bonds, with X=NC, F, Cl, CN, OH, CCH, CH3, and H. The binding energies of these complexes exhibit a second‐order dependence on the O−P distance. DFT‐SAPT binding energies correlate linearly with MP2 binding energies. The HCO2H : PXH2 complexes are stabilized by both a pnicogen bond and a hydrogen bond, resulting in greater binding energies for the HCO2H : PXH2 complexes compared to H2CO : PXH2. Neither the O−P distance across the pnicogen bond nor the O−P distance across the hydrogen bond correlates with the binding energies of these complexes. The nonlinearity of the hydrogen bonds suggests that they are relatively weak bonds, except for complexes in which the substituent X is either CH3 or H. The pnicogen bond is the more important stabilizing interaction in the HCO2H : PXH2 complexes except when the substituent X is a more electropositive group. EOM‐CCSD spin‐spin coupling constants 1pJ(O−P) across pnicogen bonds in H2CO:PXH2 and HCO2H : PXH2 complexes increase as the O−P distance decreases, and exhibit a second order dependence on that distance. There is no correlation between 2hJ(O−P) and the O−P distance across the hydrogen bond in the HCO2H : PXH2 complexes. 2hJ(O−P) coupling constants for complexes with X=CH3 and H have much greater absolute values than anticipated from their O−P distances.

DOI: 10.1021/acs.jpca.9b10681

A Conceptual DFT Study of Phosphonate Dimers: Dianions Supported by H-Bonds

A conceptual DFT study of the dissociation of anionic and neutral phosphonate dimers has been carried out. In addition, the dianion complexes have been studied in the presence of two solvents, water and tetrahydrofuran. The dissociation of the dianion complexes in the gas phase and in solution present a maximum along the reaction coordinate that is not present in the neutral–neutral and anion–neutral complexes. The principal chemical descriptors (chemical potential, reaction electronic flux, hardness, and global electrophilicity index) do not show changes in their trends along the dissociation profiles even when there is an energy maximum in the case of the anion–anion complexes.

DOI: 10.3390/cryst10030180, Download from the journal web page (OpenAccess)

Not Only Hydrogen Bonds: Other Noncovalent Interactions

In this review, we provide a consistent description of noncovalent interactions, covering most groups of the Periodic Table. Different types of bonds are discussed using their trivial names. Moreover, the new name "Spodium bonds" is proposed for group 12 since noncovalent interactions involving this group of elements as electron acceptors have not yet been named. Excluding hydrogen bonds, the following noncovalent interactions will be discussed: alkali, alkaline earth, regium, spodium, triel, tetrel, pnictogen, chalcogen, halogen, and aerogen, which almost covers the Periodic Table entirely. Other interactions, such as orthogonal interactions and π-π stacking, will also be considered. Research and applications of σ-hole and π-hole interactions involving the p-block element is growing exponentially. The important applications include supramolecular chemistry, crystal engineering, catalysis, enzymatic chemistry molecular machines, membrane ion transport, etc. Despite the fact that this review is not intended to be comprehensive, a number of representative works for each type of interaction is provided. The possibility of modeling the dissociation energies of the complexes using different models (HSAB, ECW, Alkorta-Legon) was analyzed. Finally, the extension of Cahn-Ingold-Prelog priority rules to noncovalent is proposed.

DOI: 10.1021/acs.jpca.9b10187

Are Anions of Cyclobutane Beryllium Derivatives Stabilized through Four-Center One-Electron Bonds?

High-level G4 ab initio calculations allowed us to show that C₄H₄(BeX)₄ (X = H, Cl) derivatives behave as rather efficient electron capturers due to their ability to trap the extra electron through the formation of a four-membered beryllium ring. This finding is in agreement with previous work showing the ability of highly electron-deficient atoms, such as beryllium, to lead to multicenter one-electron bonds. In our particular case, the formation of the four-center bond is characterized, in very good harmony, by different topological methods such as quantum theory of atoms in molecules (QTAIM), the electron localization function (ELF), and the noncovalent interactions (NCI) approach and is accompanied by large electron affinity values, around 300 kJ·mol–1, in the gas phase. Preliminary results may anticipate that the ability of groups of beryllium atoms to trap electrons decays on going to bigger systems.

DOI: 10.3390/molecules25051042,  Download from the journal web page (OpenAccess)

Complexes Between Adamantane Analogues B₄X₆ -X = {CH₂, NH, O ; SiH₂, PH, S} - and Dihydrogen, B₄X₆:nH₂ (n = 1–4)

In this work, we study the interactions between adamantane-like structures B₄X₆ with X = {CH₂, NH, O ; SiH₂, PH, S} and dihydrogen molecules above the Boron atom, with ab initio methods based on perturbation theory (MP2/aug-cc-pVDZ). Molecular electrostatic potentials (MESP) for optimized B₄X₆ systems, optimized geometries, and binding energies are reported for all B₄X₆:nH₂ (n = 1–4) complexes. All B₄X₆:nH₂ (n = 1–4) complexes show attractive patterns, with B₄O₆:nH₂ systems showing remarkable behavior with larger binding energies and smaller B···H₂ distances as compared to the other structures with different X.

DOI: 10.3390/cryst10020137, Download from the journal web page (OpenAccess)

Regium Bonds between Silver(I) Pyrazolates Dinuclear Complexes and Lewis Bases (N₂, OH₂, NCH, SH₂, NH₃, PH₃, CO and CNH)

A theoretical study and Cambridge Structural Database (CSD) search of dinuclear Ag(I) pyrazolates interactions with Lewis bases were carried out and the effect of the substituents and ligands on the structure and on the aromaticity were analyzed. A relationship between the intramolecular Ag–Ag distance and stability was found in the unsubstituted system, which indicates a destabilization at longer distances compensated by ligands upon complexation. It was also observed that the asymmetrical interaction with phosphines as ligands increases the Ag–Ag distance. This increase is dramatically higher when two simultaneous PH₃ ligands are taken into account. The calculated ¹⁰⁹Ag chemical shielding shows variation up to 1200 ppm due to the complexation. Calculations showed that six-membered rings possessed non-aromatic character while pyrazole rings do not change their aromatic character significantly upon complexation.

DOI: 10.1002/cphc.201900905 

Sequestration of CO₂ by Phosphatrane Molecules

The stationary points for the reaction between the CO₂ and nine different phosphatranes molecules have been characterized by means of MP2 computational methods. Two minima structures have been located: a pnicogen bonded complex where one of the oxygen atoms of CO₂ acts as electron donor and an adduct that presents a covalent P−C linkage. The corresponding transition state structure linking the two minima has also been characterized. In gas phase, the pnicogen bonded complex is more stable than the corresponding adduct except in one case. In contrast, the inclusion of the solvent effect (toluene and THF), reverts the stability, being in all cases the different adducts more stable than the pnicogen bonded complexes. The electronic properties of the systems have been analysed with the Quantum Theory of Atoms in Molecules (QTAIM) and Electron Density Shift (EDS) methods.

DOI: 10.3390/molecules24234399 (OpenAccess)

Relativistic Effects on NMR Parameters of Halogen-Bonded Complexes

Relativistic effects are found to be important for the estimation of NMR parameters in halogen-bonded complexes, mainly when they involve the heavier elements, iodine and astatine. A detailed study of 60 binary complexes formed between dihalogen molecules (XY with X, Y = F, Cl, Br, I and At) and four Lewis bases (NH₃, H₂O, PH₃ and SH₂) was carried out at the MP2/aug-cc-pVTZ/aug-cc-pVTZ-PP computational level to show the extent of these effects. The NMR parameters (shielding and nuclear quadrupolar coupling constants) were computed using the relativistic Hamiltonian ZORA and compared to the values obtained with a non-relativistic Hamiltonian. The results show a mixture of the importance of the relativistic corrections as both the size of the halogen atom and the proximity of this atom to the basic site of the Lewis base increase.

DOI: 10.1021/acs.jpca.9b08141

What Types of Noncovalent Bonds Stabilize Dimers (XCP)2, for X = CN, Cl, F, and H?

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out in search of equilibrium dimers on (XCP)₂ potential energy surfaces, for X = CN, Cl, F, and H. Five equilibrium dimers with D∞h, C∞v, Cs, C2h, and C2 symmetries exist on the (ClCP)₂ potential energy surface, four on the (FCP)₂ and (HCP)₂ surfaces, and three on the (NCCP)₂ surface. These dimers are stabilized by traditional halogen, pnicogen, and tetrel bonds, and one of them by a hydrogen bond. The binding energies of the dimers (XCP)₂ vary from 3.0 to 22.0 kJ·mol^–1, with the strongest and weakest bonds found for complexes on the (NCCP)₂ surface. The binding energies of the linear D∞h and C∞v dimers on each surface differ by no more than 1.0 kJ·mol^–1, except for (NCCP)₂, which has D∞h and C∞v complexes with binding energies of 3.0 and 11.0 kJ·mol^–1, respectively. The highly symmetric complexes with D∞h and C∞v symmetry are found on all surfaces and are the most weakly bound complexes on each surface. The structures of these dimers, the nature and strengths of charge-transfer interactions, the molecular graphs, and the molecular electrostatic potentials are useful for determining the type of intermolecular bond that stabilizes the dimers. EOM-CCSD spin–spin coupling constants ^1pJ(P–P) for complexes with P···P pnicogen bonds and D∞h symmetry are the largest coupling constants, ranging from 119 to 170 Hz. These increase with decreasing distance and follow a second-order trendline. The nature of the spin–spin coupling constants of these complexes is consistent with the type of noncovalent bond that stabilizes the dimers.

DOI: 10.1039/c9cp03694f

Modulating intramolecular chalcogen bonds in aromatic (thio)(seleno)phene-based derivatives

Intramolecular interactions have been proven to be the key to conformational control in drug-design. While chalcogen interactions have been shown to be present in certain ligands of the GK–GKRP target protein, in the present study, intramolecular chalcogen interactions through selenium are found to be even more promising since they form stronger interactions. Also, the flexibility/rigidity of the carbon backbone of the corresponding ligands is crucial in the conformational stability

DOI: 10.1002/chem.201901641 

Ternary Complexes Stabilized by Chalcogen and Alkaline-Earth Bonds: Crucial Role of Cooperativity and Secondary Noncovalent Interactions

High‐level G4 calculations show that the strength of chalcogen interactions is enhanced dramatically if chalcogen compounds simultaneously form alkaline‐earth bonds. This phenomenon is studied by exploring binary YX₂⋅⋅⋅N‐Base complexes and two types of ternary MCl₂⋅⋅⋅YX₂⋅⋅⋅N‐Base, YX₂⋅⋅⋅N‐Base⋅⋅⋅MCl₂ complexes, in which YX₂ is a chalcogen compound (Y=S, Se; X=F, Cl), the N‐Bases are sp, sp², and sp³ bases (NCH, HN=CH₂, NH₃), and MCl2 are alkaline‐earth BeCl2 or MgCl2 derivatives. Starting from the chalcogen‐bonded complexes YX₂⋅⋅⋅NH₃ and YX₂⋅⋅⋅HN=CH₂, the binding site of a new incoming alkaline‐earth bond is found, surprisingly, to depend on the nature of the halogen atom attached to the chalcogen. For the YF₂ binary complexes the association site is the F atom of the YF₂ subunit, whereas for YCl₂ it is the N atom of the nitrogen base. Regarding YX₂⋅⋅⋅NCH complexes, N is the most favorable site for an alkaline‐earth interaction in ternary complexes, regardless of which YX₂ derivative is used. The explanation relies on the interplay of all the noncovalent interactions involved: the strong cooperativity between chalcogen and alkaline‐earth bonds, and the appearance of secondary noncovalent interactions in the form of hydrogen bonds.

DOI: 10.3390/molecules24183232 (OpenAccess)

N...C and S...S Interactions in Complexes, Molecules, and Transition Structures N(CH)SX:SCO, for X = F, Cl, NC, CCH, H, and CN

Ab initio Møller–Plesset perturbation theory (MP2)/aug’-cc-pVTZ calculations have been carried out in search of complexes, molecules, and transition structures on HN(CH)SX:SCO potential energy surfaces for X = F, Cl, NC, CCH, H, and CN. Equilibrium complexes on these surfaces have C₁ symmetry, but these have binding energies that are no more than 0.5 kJ·mol^–1 greater than the corresponding C_s complexes which are vibrationally averaged equilibrium complexes. The binding energies of these span a narrow range and are independent of the N–C distance across the tetrel bond, but they exhibit a second-order dependence on the S–S distance across the chalcogen bond. Charge-transfer interactions stabilize all of these complexes. Only the potential energy surfaces HN(CH)SF:SCO and HN(CH)SCl:SCO have bound molecules that have short covalent N–C bonds and significantly shorter S^…S chalcogen bonds compared to the complexes. Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin-spin coupling constants ^1tJ(N–C) for the HN(CH)SX:SCO complexes are small and exhibit no dependence on the N–C distance, while ^1cJ(S–S) exhibit a second-order dependence on the S–S distance, increasing as the S–S distance decreases. Coupling constants ^1tJ(N–C) and ^1cJ(S–S) as a function of the N–C and S–S distances, respectively, in HN(CH)SF:SCO and HN(CH)SCl:SCO increase in the transition structures and then decrease in the molecules. These changes reflect the changing nature of the N^…C and S^…S bonds in these two systems.

DOI: 10.1021/acs.jpca.9b04144

Potential Energy Surfaces of HN(CH)SX:CO₂ for X = F, Cl, NC, CN, CCH, and H: N···C Tetrel Bonds and O···S Chalcogen Bonds

MP2/aug′-cc-pVTZ calculations have been performed in search of complexes, molecules, and transition structures on the HN(CH)SX:CO₂ potential energy surfaces, for X = F, Cl, NC, CN, CCH, and H. Complexes stabilized by traditional N···C tetrel bonds and O···S chalcogen bonds exist on all surfaces and are bound relative to the isolated monomers. Molecules stabilized by an N–C covalent bond and an O···S chalcogen bond are found when X = F, Cl, and NC, but only the HN(CH)SF:CO₂ molecule is bound. The binding energies of these complexes correlate with the O–S distance but not with the N–C distance. Binding energies of complexes rotated by 90° about the N···C tetrel bond and by 90° about the O···S chalcogen bond provide estimates of these bond energies. Charge-transfer energies across tetrel and chalcogen bonds correlate with the N–C and O–S distances, respectively. As a function of the N–C distance, equation-of-motion coupled cluster singles and doubles spin–spin coupling constants 1tJ(N–C) for complexes and transition structures and 1J(N–C) for molecules describe the evolution of the N···C tetrel bonds in the complexes and transition structures to N–C covalent bonds in the molecules. The O···S chalcogen bond gains some covalency in the transition structures and again in the molecules but does not become a covalent bond.

DOI: 10.1021/acs.jpca.9b06051

Weak Interactions Get Strong: Synergy between Tetrel and AlkalineEarth Bonds

Weak and strong noncovalent interactions such as tetrel bonds and alkaline-earth bonds, respectively, cooperate and get reinforced when acting together in ternary complexes of general formula RN··· SiH3F···MY, where MY is a Be or Mg derivative and RN is a N-containing Lewis base with different hybridization patterns. Cooperativity has been studied in the optimized MP2/aug′-cc-pVTZ ternary complexes by looking at changes on geometries, binding energies, 29Si NMR chemical shifts, and topological features according to the atoms in molecules theoretical framework. Our study shows that cooperativity in terms of energy is in general significant: more than 40 kJ/mol, and up to 83.6 kJ/mol in the most favorable case. The weakest the isolated interaction, the strongest the reinforcement in the ternary complex; in this sense, the tetrel bond is shortened enormously, between 0.3 and 0.6 Å. This dramatic reinforcement of the tetrel bond is also nicely reflected in the positive variations of the 29Si chemical shifts in all the ternary complexes. At the same time the ternary complexes are characterized by the presence of totally planar silyl group, due to the pentacoordination of the Si atom. Both the hybridization of the N base and the geometry imposed by the alkaline-earth ligands have a strong influence on the binding energies, as they modify the donor ability of N and the Lewis acid character of the alkaline-earth metal.

DOI: 10.1039/c9cp03463c

Systematic behaviour of electron redistribution on formation of halogen-bonded complexes B...XY, as determined via XY halogen nuclear quadrupole coupling constants

Equilibrium nuclear quadrupole coupling constants associated with the di-halogen molecule XY in each of 60 complexes B⋯XY (where B is one of the Lewis bases N2, CO, HCN, H2O, H2S, HCCH, C2H4, PH3, NH3 or (CH3)3N and XY is one of the di-halogens Cl2, BrCl, Br2, ICl, IBr or I2) have been calculated ab initio. The Townes–Dailey model for interpreting the changes in the coupling constants when XY enters the complex was used to describe the electron redistribution in the di-halogen molecule in terms of the fraction δi of an electron transferred from the Lewis base B to atom X and the fraction δp of an electron transferred simultaneously from atom X to atom Y. Systematic relationships between the δi values for the six series are established. It is shown that, in reasonable approximation, δi decays exponentially as the first ionisation energy IB of the Lewis base B increases, that is δi = A exp(−bIB). It is concluded from the results for the series B⋯BrCl, B⋯Br2, B⋯ICl, B⋯IBr and B⋯I2 that the coefficients A and b in regression fits to the corresponding logarithmic version ln(δi) = ln(A) − b(IB) of the equation are not strongly dependent on either the halogen atom X directly involved in the halogen bond in B⋯XY or, for a given X, on the nature of Y. The behaviour of PH3 as a Lewis base appears to be anomalous. Values of δi and δp calculated by the quantum theory of atoms-in-molecules and natural bond orbital methodologies are very close to those from application of the Townes–Dailey approach described.

DOI: 10.1016/j.cplett.2019.05.044

Exploring N…C tetrel and O…S chalcogen bonds in HN(CH)SX:OCS systems, for X = F, NC, Cl, CN, CCH, and H

Ab initio MP2/aug’-cc-pVTZ calculations were carried out to investigate HN(CH)SX:OCS potential energy surfaces. Bound equilibrium complexes stabilized by N…C tetrel bonds and O…S chalcogen bonds have been found on all HN(CH)SX:OCS surfaces, but HN(CH)SF:OCS is the only bound molecule. Charge-transfer interactions are consistent with the nature of the intermolecular bonds in these complexes. Spin-spin coupling constants ^1tJ(N-C) for HN(CH)SF:OCS are consistent with the changing nature of the N…C bond as the complex passes through the transition state to become a molecule. ^1cJ(O-S) describes an intermolecular chalcogen bond in the complex, transition structure, and molecule.

DOI: 10.1002/cphc.201900354

Understanding Regium Bonds and their Competition with Hydrogen Bonds in Au₂:HX Complexes

A theoretical study of the regium and hydrogen bonds (RB and HB, respectively) in Au₂:HX complexes has been carried out by means of CCSD(T) calculations. The theoretical study shows as overall outcome that in all cases the complexes exhibiting RB are more stable that those with HB. The binding energies for RB complexes range between −24 and −180 kJ ⋅ mol⁻¹, whereas those of the HB complexes are between −6 and −19 kJ ⋅ mol⁻¹. DFT‐SAPT also indicated that HB complexes are governed by electrostatics, but RB complexes present larger contribution of the induction term to the total attractive forces. ¹⁹⁷Au chemical shifts have been calculated using the relativistic ZORA Hamiltonian.

DOI: 10.1080/00268976.2018.1521012

Complexes between H2 and neutral oxyacid beryllium derivatives. The role of angular strain

The complexes between 14 different Be-salts of oxyacids from groups 13 to 16 with one and two molecules of H2 have been investigated by means of the MP2/aug-cc-pVTZ ab initio molecular orbital theory method, which was found to be reliable for the treatment of these weakly bound species. The main conclusion is that these Be-salts yield rather stable complexes with dihydrogen, with binding energies one order of magnitude larger than other typical H2 complexes reported in the literature. This strong binding is shown to be due to an enhancement of the electron-deficient nature of Be when attached to an oxyacid moiety, which depends more on the type of coordination of the central atom of the oxyacid moiety. The formation of these complexes is followed by a significant lengthening of the H2internuclear distance and a concomitant red-shift of the H–H stretching frequency, which becomes a good indicator of the strength of the interaction. The charge shifting from the bonding region of the H2molecule to the interboundary Be···H2 region is the physical phenomenon behind the stability of these complexes. Accordingly, the most important contributor to this stability is the inductive term, followed by the electrostatic interactions. The ability of Be to bind H2 is enhanced by the angular arrangement of the O–Be–O electron-acceptor group.

DOI: 10.1080/00268976.2018.1512726

Pnicogen bonds in complexes with CO and CS: differentiating properties

Ab initio MP2/aug’-cc-pVTZ calculations have been performed on pnicogen-bonded complexes with CO and CS as electron-pair donors to PH2X, for X = F, NC, OH, CN, CCH, and H. CO:PH2X and OC:PH2X complexes are stabilised by traditional pnicogen bonds. CS is an electron-pair donor through its in-plane π system to four PH2X molecules. It forms C··· P phosphorus-shared bonds with some ion-pair character with PH2F, PH2(OH-Z), and PH2(OH-E), and traditional pnicogen bonds with all PH2X except PH2F. C-O and C-S stretching frequencies are blue-shifted for C···P pnicogen bonds, and red-shifted for O···P and S···P bonds. EOM-CCSD spin-spin coupling constants 1pJ(P-C) for OC:PH2X and 1pJ(P-O) for CO:PH2X are characteristic of complexes stabilised by traditional pnicogen bonds. Coupling constants 1pJ(P-C) as a function of the P-C distance for SC:PH2X illustrate the evolution of the C···P pnicogen bond. They increase as the P-C distance decreases in complexes with traditional bonds, reach a maximum for SC:PH2OH transition structures as the P-C distance further decreases and the bonds gain phosphorus-shared character, and then change sign and continue to decrease as the P-C distance further decreases and the phosphorus-shared pnicogen bonds gain ion-pair character. They approach the values of 1J(P-C) for the cation (H2PCS)+.

DOI: 10.1021/acs.jpca.9b00553

Can a Cl−H···F Hydrogen Bond Replace a Cl···F Halogen Bond? H2XP:ClY:ZH versus H2XP:ClY:HZ for Y, Z = F, Cl

Ab initio MP2/aug’-cc-pVTZ (where MP2 is Møller–Plesset perturbation theory) calculations have been carried out on four series of complexes, H2XP:ClF:HCl, H2XP:ClF:HF, H2XP:ClCl:HF, and H2XP:ClCl:HCl, to answer the question raised in the title of this paper. When X is F or Cl, binary complexes containing a P(V) molecule hydrogen bonded to an acid are found on all potential surfaces except H2ClP:ClF:HF, where an ion–pair complex exists. Ion–pair complexes also result from the optimization of H2XP:ClF:HF for X = NC, CN, and H. Changing the central molecule from ClF to ClCl has a dramatic effect on the nature of the optimized complexes when the substituents are NC, CN, and H. On the potential surfaces H2XP:ClCl:FH for X = NC and CN, open ternary complexes stabilized by a pnicogen bond and a hydrogen bond are found. Optimization of H3P:ClCl:FH leads to an ion pair. For H2(NC)P:ClCl:HCl and H2(CN)P:ClCl:HCl, cyclic ternary complexes stabilized by pnicogen, halogen, and hydrogen bonds result from optimization. Optimization of H3P:ClCl:HCl leads to a reaction in which H2ClP and a second HCl molecule are formed, and the resulting cyclic ternary complex is stabilized by two hydrogen bonds and a pnicogen bond. Thus, the type of complex resulting from the optimization of the starting ternary complex H2XP:ClY:HZ depends on the nature of the central molecule ClF or ClCl, the terminal molecule HCl or HF, and the substituent X. It is not possible to simply turn around the terminal HZ molecule in complexes H2XP:ClF:ZH for Z = F and Cl to give H2XP:ClF:HZ, thereby replacing a halogen bond by a hydrogen bond. Complexes H2XP:ClCl:HZ for X = NC and CN are stable complexes, but the corresponding halogen-bonded complexes H2XP:ClCl:ZH are not.

DOI: 10.1007/s00214-019-2424-3

Gas‑phase reactivity tuned through the interaction with alkaline‑earth derivatives

The cooperativity between MX2:XH alkaline-earth bonds and XH:NH3 hydrogen bonds (M = Mg, Ca; X = F, Cl) was investigated at the G4 level of theory. The cooperativity between these two non-covalent linkages is extremely large, to the point that the increase in their bond dissociation enthalpies may be as large as 240%. More importantly, the weaker the interaction, the larger the increase, so in some cases the linkage that stabilizes the most is the alkaline-earth bond, whereas in others is the hydrogen bond. In all cases, the formation of the MX2:XH:NH3ternary complex is followed by a spontaneous proton transfer, very much as previously found for the Be-containing analogues. Similarly, MX2:FCl:NH3 complexes evolve from a chlorine-shared ternary complex (MX2F···Cl···NH3) or from an ion pair (MX2F−···NH3Cl+) if M = Ca. Although F is the only halogen without σ-hole, MgCl2 derivatives induce the appearance of a σ-hole on it, though less deep than those induced by BeCl2. We have also studied whether Mg and Ca bond-containing complexes MR2:FY (R = H, F, Cl; Y = NH2, OH, F, Cl) may react to form radicals, as it has been found for the Be-containing analogues. These interactions provoke a drastic decrease in the F–Y bond dissociation enthalpy, very much as the one reported for the corresponding Be-analogues, to the point that in some cases the formation of the corresponding MR2F• + Y· radicals becomes exothermic. Hence, the general conclusion of this study is that Mg or Ca derivatives give place to similar or even larger perturbations on the electron density than those induced by Be, a result not easily predictable.

DOI: 10.3390/inorganics7030035 (OpenAccess)

Non-Covalent Interactions Involving Alkaline-Earth Atoms and Lewis Bases B: An ab Initio Investigation of Beryllium and Magnesium Bonds, B···MR₂ (M = Be or Mg, and R = H, F or CH₃)

Geometries, equilibrium dissociation energies (De), intermolecular stretching, and quadratic force constants (kσ) determined by ab initio calculations conducted at the CCSD(T)/aug-cc-pVTZ level of theory, with De obtained by using the complete basis set (CBS) extrapolation [CCSD(T)/CBS energy], are presented for the B···BeR₂ and B···MgR₂ complexes, where B is one of the following Lewis bases: CO, H₂S, PH₃, HCN, H₂O or NH₃, and R is H, F or CH₃. The BeR₂ and MgR₂ precursor molecules were shown to be linear and non-dipolar. The non-covalent intermolecular bond in the B···BeR₂ complexes is shown to result from the interaction of the electrophilic band around the Be atom of BeR₂ (as indicated by the molecular electrostatic potential surface) with non-bonding electron pairs of the base, B, and may be described as a beryllium bond by analogy with complexes such as B···CO₂, which contain a tetrel bond. The conclusions for the B···MgR₂ series are similar and a magnesium bond can be correspondingly invoked. The geometries established for B···BeR2 and B···MgR2 can be rationalized by a simple rule previously enunciated for tetrel-bonded complexes of the type B···CO₂. It is also shown that the dissociation energy, De, is directly proportional to the force constant, kσ, in each B···MR₂ series, but with a constant of proportionality different from that established for many hydrogen-bonded B···HX complexes and halogen-bonded B···XY complexes. The values of the electrophilicity, EA, determined from the De for B···BeR₂ complexes for the individual Lewis acids, A, reveal the order A = BeF₂ > BeH₂ > Be(CH₃)₂—a result that is consistent with the −I and +I effects of F and CH₃ relative to H. The conclusions for the MgR₂ series are similar but, for a given R, they have smaller electrophilicities than those of the BeR₂ series. A definition of alkaline-earth non-covalent bonds is presented. 

DOI: 10.1016/j.cplett.2019.02.016

Probing C⋯S chalcogen bonds in complexes SC:SHX, for X = NO₂, NC, F, Cl, CN, CCH, and NH₂

MP2/aug’-cc-pVTZ calculations have been carried out to determine the structures and binding energies of complexes SC:SHX, for X = NO₂, NC, F, Cl, CN, CCH, and NH₂. Complexes with traditional chalcogen bonds exist on all surfaces except for X = F and Cl, while complexes with sulfur-shared chalcogen bonds exist when X = F, NO₂, NC, and Cl. The transition structures which present the barriers to converting one equilibrium structure to another on surfaces with double minima have been obtained. Charge-transfer energies, AIM parameters, NMR chemical shieldings, and EOM-CCSD spin-spin coupling constants across chalcogen bonds characterize these complexes.

DOI: 10.1039/c8cp07542e (Open Access)

Cations brought together by hydrogen bonds: the protonated pyridine–boronic acid dimer explained

According to the Cambridge Structural Database, protonated pyridine–boronic acid dimers exist in the solid phase, apparently defying repulsive coulombic forces. In order to understand why these cation–cation systems are stable, we carried out M06-2X/6-311++G(3df,2pd) electronic structure calculations and used a set of computational tools (energy partitioning, topology of the electron density and electric field maps). The behavior of the charged dimers was compared with the corresponding neutral systems, and the effect of counterions (Br− and BF₄−) and the solvent (PCM model) on the binding energies has been considered. In the gas-phase, the charged dimers present positive binding energies but are local minima, with a barrier (16–19 kJ mol⁻¹) preventing dissociation. Once the environment is included via solvent effects or counterions, the binding energies become negative; remarkably, the strength of the interaction is very similar in both neutral and charged systems when a polar solvent is considered. Essentially, all methods used evidence that the intermolecular region where the HBs take place is very similar for both neutral and charged dimers. The energy partitioning explains that repulsion and electrostatic terms are compensated by the desolvation and exchange terms in polar solvents, thus giving stability to the charged dimer.

DOI: 10.1063/1.5085281

A chalcogen-bonded complex H₃N· · ·S=C=S formed by ammonia and carbon disulfide characterised by chirped-pulse, broadband microwave spectroscopy

Ground-state rotational spectra were observed for ten symmetric-top isotopologues H₃N⋯S=C=S, H₃N⋯³⁴S=C=S, H₃N⋯S=C=³⁴S, H₃N⋯S=¹³C=S, H₃¹⁵N⋯S=C=S, H₃¹⁵N⋯³⁴S=C=S, H₃¹⁵N⋯S=C=³⁴S, H₃¹⁵N⋯S=¹³C=S, H₃¹⁵N⋯³³S=C=S, and H₃¹⁵N⋯S=C=³³S, the first five in their natural abundance in a mixture of ammonia and carbon disulphide in argon and the second group with enriched ¹⁵NH₃. The four asymmetric-rotor isotopomers H₂DN⋯S=C=S, H₂DN⋯³⁴S=C=S, H₂DN⋯S=C=³⁴S, and HD₂N⋯S=C=S were investigated by using a sample composed of ND₃mixed with CS₂. Rotational constants, centrifugal distortion constants, and 33S nuclear quadrupole coupling constants were determined from spectral analyses and were interpreted with the aid of models of the complex to determine its symmetry, geometry, one measure of the strength of the intermolecular binding, and information about the subunit dynamics. The complex has C3v symmetry, with nuclei in the order H₃N⋯S=C=S, thereby establishing that the non-covalent interaction is a chalcogen bond involving the non-bonding electron pair of ammonia as the nucleophile and the axial region near one of the S atoms as the electrophile.The small intermolecular stretching force constant kσ = 3.95(5) N m−1 indicates a weak interaction and suggests the assumption of unperturbed component geometries on complex formation. A simple model used to account for the contribution of the subunit angular oscillations to the zero-point motion leads to the intermolecular bond length r(N⋯S) = 3.338(10) Å.

DOI: 10.1016/bs.adioch.2018.10.003

The beryllium bond

This chapter describes the strong and weak interactions of beryllium to form covalent bonds and beryllium bonds. We start from the challenging description of bonding in the beryllium dimer, trimer and larger clusters, followed by a brief review of other beryllium-beryllium bonds and organoberyllium compounds. Thereafter, the beryllium bond is defined and characterized in binary complexes, and it's very strong effects on the acid/base properties of the attached molecules reported. The last sections discuss the effect of the beryllium bond on reactivity and cooperative effects of compounds containing several beryllium bonds.

DOI: 10.1002/jhet.3331

Interaction of N‐Heterocyclic Carbenes and Simple Carbenes with Small Molecules (One to Three Atoms) Excluding Metals: Formation of Covalent C–X Bonds

This review reports the reactivity of carbenes (>C:), both N‐heterocyclic and simpler ones, with 29 small molecules to form C–X bonds and some weak interactions. The structures thus obtained belong to many functional groups proving the characteristic reactivity of N‐heterocyclic carbenes. Particular attention has been paid to X‐ray structures that are useful for theoretical calculations.

DOI: 10.1039/c8cp06908e

Modulating the intrinsic reactivity of molecules through non-covalent interactions

DOI: 10.3390/molecules24020308

Cooperative Effects in Weak Interactions: Enhancement of Tetrel Bonds by Intramolecular Hydrogen Bond

A series of silyl and germanium complexes containing halogen atoms (fluorine and chlorine atoms) and exhibiting tetrel bonds with Lewis bases were analyzed by means of Møller-Plesset computational theory. Binding energies of germanium derivatives were more negative than silicon ones. Amongst the different Lewis bases utilized, ammonia produced the strongest tetrel bonded complexes in both Ge and Si cases, and substitution of the F atom by Cl led to stronger complexes with an ethylene backbone. However, with phenyl backbones, the fluorosilyl complexes were shown to be less stable than the chlorosilyl ones, but the opposite occurred for halogermanium complexes. In all the cases studied, the presence of a hydroxyl group enhanced the tetrel bond. That effect becomes more remarkable when an intramolecular hydrogen bond between the halogen and the hydrogen atom of the hydroxyl group takes places.

DOI: 10.1002/cphc.201800878

Energetic, Topological and Electric Field Analyses of Cation-Cation Nucleic Acid Interactions in Watson-Crick Disposition

A theoretical study of the effect of the diprotonation on the nucleic acid bases (A : U, A : T and G : C) in Watson‐Crick conformation has been carried out by means of DFT computational methods in vacuum. In addition, the corresponding neutral and monoprotonated binary complexes have been considered. Most of the diprotonated species studied are stable, even though the binding energy is positive due to the overall repulsive electrostatic term. Local electrostatic attractive forces in the regions of hydrogen bonds (HBs) are responsible for equilibrium geometries, as shown by the electric field lines connecting the electrophilic and nucleophilic sites involved in the HB interactions. Secondary electrostatic effects also affect the assembling of the nucleic acid complexes in either neutral or cationic form. In particular, the electric field lines flowing from electrophilic sites in one base to nucleophilic sites in the other reinforce the linking between them. Hence, when the nucleophilic site concerns the free lone pair of the heteroatom involved in the HB interaction as acceptor, the HB distance shortens. However, if the free lone pair of the HB acceptor interacts with an electrophilic site in the same molecule, the HB distance elongates, weakening the HB interaction. The topological analysis of the electron density distribution in HB regions indicates that neutral, monoprotonated and diprotonated complexes show no differences in the nature of their HB's.

DOI: 10.1039/C8RA06031B

A DFT study on nanocones, nanotubes (4,0), nanosheets and fullerene C₆₀ as anodes in Mg-ion batteries

In this article, we studied the interactions between Mg atom and Mg2+ ion and four nanostructures, including a nanocone, nanotube (4,0), nanosheet, and C60 nanocage, to obtain the cell voltages (V) for Mg-ion batteries (MIBs). Total energy, geometry optimization, frontier molecular orbital (FMO) and density of states (DOS) analyses have been performed using the ωB97XD level of theory and the 6-31G(d) basis set. The DFT calculations clarified that the changes in energy adsorption between Mg2+ ion and the nanostructures, Ead, are in the order tube > cone > sheet > cage. However, Vcell for the nanocone is the highest. The changes in Vcell of the MIBs are in the order cone > tube > sheet > cage. This study theoretically considers the possibilities of Mg as an anode in batteries due to its high Vcell values.

DOI: 10.1016/j.comptc.2018.12.014

Lin@Tetracyanoethylene (n = 1–4) systems: Lithium salt vs lithium electride

Electrides are interesting and promising materials with cavity-trapped electrons which can be used as source of electron donor in different systems. Hereby, we have explored the possible formation of electride materials based on tetracyanoethylene (TCNE) backbone at MP2 computational level. This is achieved by systematic addition of up to four Li atoms to TCNE backbone. Our results predict high thermodynamic stability in the Lin@TCNE (n = 1–4) systems. Moreover, based on the evaluation of four criteria, non-nuclear attractor (NNA), electron localization function (ELF), electron density laplacian, and non-linear optical (NLO), TCNE-Li1 and TCNE-Li2 and TCNE-Li4 species are conventional donor-acceptor systems (lithium salt). In contrast, the TCNE-Li3 species can be introduced as lithium electride with cavity-trapped electrons. Therefore, Li:TCNE ratio is very significant factor to provide species with electride feature through the addition of Li atoms to TCNE backbone.

DOI: 10.3390/inorganics6040110  (Open Access PDF)

Solvent and Substituent Effects on the Phosphine + CO₂ Reaction

A theoretical study of the substituent and solvent effects on the reaction of phosphines with CO₂ has been carried out by means of Møller-Plesset (MP2) computational level calculations and continuum polarizable method (PCM) solvent models. Three stationary points along the reaction coordinate have been characterized, a pre-transition state (TS) assembly in which a pnicogen bond or tetrel bond is established between the phosphine and the CO₂ molecule, followed by a transition state, and leading finally to the adduct in which the P–C bond has been formed. The solvent effects on the stability and geometry of the stationary points are different. Thus, the pnicogen bonded complexes are destabilized as the dielectric constant of the solvent increases while the opposite happens within the adducts with the P–C bond and the TSs trend. A combination of the substituents and solvents can be used to control the most stable minimum.

DOI: 10.1002/cphc.201800518

Enhancement of Thermodynamic Gas‐Phase Acidity and Basicity of Water by Means of Secondary Interactions

A series of A⋅water, B⋅water complexes (A=acid, B=base) are studied at the G4 level of theory to show that water acidity or basicity can be modulated by non‐covalent interactions. Protic and non‐protic acids interacting with water form hydrogen bonds or other kinds of non‐covalent interactions, respectively, that may dramatically change the acidity of water up to almost 360 kJ ⋅ mol−1 in terms of enthalpy. Similarly, hydrogen bonds responsible for the interaction between typical small nitrogen‐containing Lewis bases and water can enhance the proton affinity of water by almost 300 kJ ⋅ mol−1. Our results reveal that these large enhancements are linearly related with the binding energy of the charged complexes, and are determined by the Lewis acid−base properties of the molecule involved in the interaction, allowing a quite precise modulation of the corresponding acid−base properties of water.

DOI: 10.1021/acs.jpclett.8b02339

Competing Dispersive Interactions: From Small Energy Differences to Large Structural Effects in Methyl Jasmonate and Zingerone

Modern structural studies of biologically relevant molecules require an exhaustive interplay between experiment and theory. In this work, we present two examples where a poor choice of the theoretical method led to a misinterpretation of experimental results. We do that by performing a rotational spectroscopy study on two large and flexible biomolecules: methyl jasmonate and zingerone. The results show the enormous potential of rotational spectroscopy as a benchmark to evaluate the performance of theoretical methods.

DOI: 10.1039/c8dt01679h

Complexes between neutral oxyacid beryllium salts and dihydrogen: a possible way for hydrogen storage?

Accurate ab initio calculations reveal that oxyacid beryllium salts yield rather stable complexes with dihydrogen. The binding energies range between −40 and −60 kJ mol⁻¹ for 1 : 1 complexes, remarkably larger than others previously reported for neutral H₂ complexes. The second H₂ molecule in 1 : 2 complexes is again strongly bound (between −18 and −20 kJ mol⁻¹). The incoming H₂ molecules in 1 : n complexes (n = 3–6) are more weakly bound, confirming the preference of Be for tetracoordinated arrangements.

DOI: 10.1016/j.cplett.2018.08.027

Using protonation to change a Cl^...N halogen bond in N-Base:ClOH complexes to a Cl^...O halogen bond

Ab initio MP2/aug′-cc-pVTZ calculations demonstrate that protonation of N-Base:ClOH complexes at O leads to complexes (N-Base-Cl)+:OH2, as a traditional N^...Cl halogen bond is replaced by a chlorine-transferred Cl^...O bond. For a fixed base, the binding energies of the latter complexes are more than an order of magnitude greater than the former. As a function of the Cl-N distance, EOM-CCSD Cl-N coupling constants show the evolution of the traditional halogen bond in the N-Base:ClOH complexes, to a chlorine-shared halogen bond in N2:ClOH2+, to a covalent bond in (N-Base-Cl)+:OH2. 1J(Cl-N) values for these complexes resemble 1J(Cl-N) for the ions (N-Base-Cl)+.

DOI: 10.3390/molecules23092250  (Open Access)

An Ab Initio Investigation of the Geometries and Binding Strengths of Tetrel-, Pnictogen-, and Chalcogen-Bonded Complexes of CO2, N2O, and CS2 with Simple Lewis Bases: Some Generalizations

Geometries, equilibrium dissociation energies (De), and intermolecular stretching, quadratic force constants (kσ) are presented for the complexes B⋯CO2, B⋯N2O, and B⋯CS2, where B is one of the following Lewis bases: CO, HCCH, H2S, HCN, H2O, PH3, and NH3. The geometries and force constants were calculated at the CCSD(T)/aug-cc-pVTZ level of theory, while generation of De employed the CCSD(T)/CBS complete basis-set extrapolation. The non-covalent, intermolecular bond in the B⋯CO2 complexes involves the interaction of the electrophilic region around the C atom of CO2 (as revealed by the molecular electrostatic surface potential (MESP) of CO2) with non-bonding or π-bonding electron pairs of B. The conclusions for the B⋯N2O series are similar, but with small geometrical distortions that can be rationalized in terms of secondary interactions. The B⋯CS2 series exhibits a different type of geometry that can be interpreted in terms of the interaction of the electrophilic region near one of the S atoms and centered on the C∞ axis of CS2 (as revealed by the MESP) with the n-pairs or π-pairs of B. The tetrel, pnictogen, and chalcogen bonds so established in B⋯CO2, B⋯N2O, and B⋯CS2, respectively, are rationalized in terms of some simple, electrostatically based rules previously enunciated for hydrogen- and halogen-bonded complexes, B⋯HX and B⋯XY. It is also shown that the dissociation energy De is directly proportional to the force constant kσ, with a constant of proportionality identical within experimental error to that found previously for many B⋯HX and B⋯XY complexes.

DOI: 10.1002/cphc.201800217

Complexes of O=C=S with Nitrogen Bases: Chalcogen Bonds, Tetrel Bonds, and Other Secondary Interactions

Ab initio MP2/aug’‐cc‐pVTZ calculations have been carried out to investigate chalcogen‐bond formation through the σ‐hole at S and tetrel‐bond formation through the π‐hole at C in complexes of OCS with a series of nitrogen bases. The binding energies of chalcogen‐ and tetrel‐bonded complexes with the sp‐hybridized bases correlate exponentially with the N−S and N−C distances, respectively. The presence of secondary interactions between an N−H or C−H group of an sp2‐hybridized base and OCS in chalcogen‐bonded complexes decreases the correlation between binding energies and the N−S distance. These secondary interactions are stronger in the tetrel‐bonded complexes with the sp2 bases, particularly in the isomers of OCS:imidazole and OCS : N2H2, where they may be described as distorted N−H⋅⋅⋅O or N−H⋅⋅⋅S hydrogen bonds. Charge‐transfer interactions are consistent with the nature of the primary and secondary interactions in these complexes. The in‐plane OCS bending frequencies are blue‐shift in the chalcogen‐bonded complexes, and red‐shifted in the tetrel‐bonded complexes. EOM‐CCSD spin‐spin coupling constants 1cJ(N4−S) across chalcogen bonds have absolute values less than 9.0 Hz, while the two‐bond coupling constants 2cJ(N4−C) do not exceed 4.0 Hz. These are greater in absolute value that the one‐bond coupling constants 1tJ(N4−C) across tetrel bonds that are less than 0.5 Hz at much shorter N−C distances.

DOI: 10.1039/C8NJ01879K

A theoretical study of perovskites related to CH₃NH₃PbX₃ (X = F, Cl, Br, I)

The bond dissociation energies of MAPI (CH₃NH₃PbI₃) and related perovskites (with F, Cl, Br instead of I and with bases other than methylamine) have been calculated using a simplified model consisting of a corner of the perovskite (PbX₃−) and the methyl ammonium (CH₃NH₃+) and other protonated bases. The values obtained show that besides the size (related to the tolerance factor), the energy of the interaction should be considered. Using relativistic corrections (ADF), the ¹H, ¹³C, ¹⁵N and ²⁰⁷Pb absolute shieldings were calculated and transformed into chemical shifts by empirical equations established here. The ^₁₃C and ¹⁵N light nuclei were well reproduced by the “corner” model but the ²⁰⁷Pb chemical shifts need a large correction factor owing to the fact that lead in the perovskites is surrounded by six iodine atoms instead of the “corner's” three atoms.

DOI: 10.1080/00268976.2018.1488006

Analysis of the interactions in FCCF:(H₂O) and FCCF:(H₂O)₂ complexes through the study of their indirect spin–spin coupling constants

A theoretical study of FCCF:(H₂O)_n complexes, with n = 1 and 2, has been carried out by means of ab initio computational methods. Three kinds of interactions are observed in the complexes: H···π and H···F hydrogen bonds and O···FC tetrel bonds. The indirect spin–spin coupling constants have been calculated at the CCSD/aug-cc-pVTZ-J computational level. Special attention has been paid to the dependence of the different intramolecular coupling constants in FCCF on the distance between the coupled nuclei and the presence or absence of water molecules. The exceptional sensitivity shown by these coupling constants to the presence of water molecules is quite notorious and can provide information on the bonding structure of the molecule.

DOI: 10.1039/c8cp03217c

Binding indirect greenhouse gases OCS and CS₂ by nitrogen heterocyclic carbenes (NHCs)

Carbon disulfide (CS₂) and carbonyl sulfide (OCS) are indirect greenhouse gases that can be effectively trapped by classical, abnormal and remote nitrogen heterocyclic carbenes (NHCs), according to high level ab initiocalculations. The process is described through a reaction profile involving two minima, a non-covalent complex and a covalently bound product, connected by a single transition state. Both CS₂ and OCS react towards NHCs in a similar way, forming a new C–C bond and leading to very stable products with feasible barriers in many cases, although they vary significantly depending on the NHC structure. The barriers are larger than those reported for CO₂, oscillating from barrierless processes up to a maximum of 57.9 kJ mol⁻¹, whereas the products are more stabilized than those incorporating CO2. The lowest barriers for the CS₂ + NHC reactions correspond to the largest C–C distances in the products, unlike the CO₂ case. Remarkably, the most favored reactions, which are those involving a remote NHC, do not exhibit the highest interaction energies at the TS, but low distortion energy values of the OCS/CS2 moieties. The decomposition of the interaction energy allowed to confirmed that in fact the remote carbene is the less favored one in terms of the electrostatic and exchange terms. Substitution in CO₂ of O by a more polarizable atom such as S have a great influence on the balance between the steric plus orbital interaction and the kinetic energy terms, thus making the products between NHCs and OCS/CS₂ more stable. Both OCS and CS₂ become better charge acceptors than CO₂ on going from the starting complexes to the products.

DOI: 10.1002/cphc.201800292

Be‐ and Mg‐Based Electron and Anion Sponges

By using G4(MP2) high‐level ab initio methods, we show that Be and Mg derivatives of cyclopropane exhibit very large electron and anion affinities, reflecting the electron‐deficient nature of the −BeX and −MgX (X=CH3, F, Cl, CN) substituents. In particular, these compounds present electron affinities among the largest reported for neutral closed‐shell systems. Their anion affinities are also among the largest reported for single neutral molecules, indeed higher than the 1,8‐diBeX‐naphthalene (X=F, Cl, CN) derivatives, recently shown to behave as anion sponges. Quite unexpectedly, our results indicate that the intrinsic anion affinity of the Mg‐containing compounds is higher than that of the Be‐containing analogs

DOI: 10.1039/c8nj00470f

Strengths of non-covalent interactions in hydrogen-bonded complexes B···HX and halogen-bonded complexes B···XY (X, Y = F, Cl): an ab initio investigation

DOI: 10.1007/s00214-018-2274-4

Intramolecular magnesium bonds in malonaldehyde-like systems: a critical view of the resonance-assisted phenomena

Through the use of high-level G4-theory calculations, we have investigated the structure, stability, and bonding of a set of Mg derivatives formed by replacing the –OH group of malonaldehyde or only the hydrogen atom of this group by a –MgH group. To give insight into the resonance-assisted phenomenon, which might be involved in the stabilization of these compounds, we also included the corresponding saturated analogs in our survey. The effect of the rigidity of the molecular framework was considered by analyzing the Mg derivatives of (Z)-4-(hydroxymethylene)cyclobut-2-enone, obtained through the same substitutions mentioned above. The effect of replacing the carbonyl group by an imino group was also contemplated. In all cases, the global minimum is a cyclic conformer stabilized through the formation of rather strong intramolecular magnesium bonds. The strength of these interactions is directly related with the intrinsic basicity of the carbonyl group (or the imino group) and the intrinsic acidity of the –MgH group, rather than with a resonance-assisted phenomenon. As a matter of fact, for all the investigated systems, the conclusion is that resonance in the cyclic conformer is directly correlated with the strength of the intramolecular magnesium bond, and not vice versa. Interestingly, the strength and characteristics of these interactions for these Mg-containing derivatives are very similar to those of the corresponding Be-containing analogs.

DOI: 10.5562/cca3258  (OpenAccess)

The Hydrogen-bond Basicity of Carbenes

We have evaluated theoretically in the gas-phase (MP2/aug-cc-pVTZ) the hydrogen-bond basicity of simple carbenes and vinylidenes and compared them to the corresponding nitrogen and oxygen derivatives using HF as Lewis acid. These values fit conveniently with B values only if sp3 and sp2 atoms are treated separately, which is a consequence of the gas-phase (calculated) vs. solution (measured) effects.

DOI: 10.1002/cphc.201701240

Trapping One Electron between Three Beryllium Atoms: Very Strong One-Electron Three-Center Bonds

The ability of a set of beryllium‐substituted cyclohexane derivatives to trap electrons was determined by evaluating their electron affinities at the G4(MP2) level of theory. The nature of bonding and the effect of the different substituents attached to beryllium were studied by different computational methods (quantum theory of atoms in molecules, electron localization function, natural bond orbital, and analysis of the spin density), revealing the existence of a one‐electron/Be3 cyclic bonding in trisubstituted species. This peculiar bond is the key for the high electron affinity values found in the tri‐BeX derivatives (X=F, Cl, CN), such as the triberyllium cyano derivatives of cyclohexane, reaching values of 294 kJ mol−1, only marginally smaller than the values reported for tetracyanoethylene (305 kJ mol−1) and for some fullerenes (306 kJ mol−1).

DOI: 10.1080/00268976.2018.1433337

Weak interactions and cooperativity effects on disiloxane: a look at the building block of silicones

The behaviour of disiloxane 1 towards a set of Lewis acids (LA) and Lewis bases (LB) forming complexes through its oxygen and silicon atoms, respectively, was studied at the MP2/aug′-cc-pVTZ level of theory, exploring a wide variety of non-covalent interactions. Disiloxane is a moderate electron acceptor and a good electron donor, exhibiting in the latter case binding energies up to almost −100 kJ/mol with BeCl₂. Cooperativity effects were also analysed by looking at ternary 1:LA:LB complexes. Shorter intermolecular distances than in the corresponding binary complexes and a negative contribution of the three-body term to the binding energy indicate that the non-covalent interactions allowed by disiloxane through its acid and basic centres cooperate between them to reinforce both donor–acceptor pairs. These effects are particularly strong in complexes involving beryllium and triel bonds, but are also relevant for complexes containing hydrogen bonds.

DOI: 10.3390/molecules23040906 (OpenAccess)

Complexes of CO₂ with the Azoles: Tetrel Bonds, Hydrogen Bonds and Other Secondary Interactions

Ab initio MP2/aug’-cc-pVTZ calculations have been performed to investigate the complexes of CO₂ with the azoles pyrrole, pyrazole, imidazole, 1,2,3- and 1,2,4-triazole, tetrazole and pentazole. Three types of complexes have been found on the CO₂:azole potential surfaces. These include ten complexes stabilized by tetrel bonds that have the azole molecule in the symmetry plane of the complex; seven tetrel-bonded complexes in which the CO₂ molecule is perpendicular to the symmetry plane; and four hydrogen-bonded complexes. Eight of the planar complexes are stabilized by Nx···C tetrel bonds and by a secondary interaction involving an adjacent Ny-H bond and an O atom of CO₂. The seven perpendicular CO₂:azole complexes form between CO₂ and two adjacent N atoms of the ring, both of which are electron-pair donors. In three of the four hydrogen-bonded complexes, the proton-donor Nz-H bond of the ring is bonded to two C-H bonds, thereby precluding the planar and perpendicular complexes. The fourth hydrogen-bonded complex forms with the strongest acid pentazole. Binding energies, charge-transfer energies and changes in CO₂ stretching and bending frequencies upon complex formation provide consistent descriptions of these complexes. Coupling constants across tetrel bonds are negligibly small, but ^2hJ(Ny-C) across Nz-H···C hydrogen bonds are larger and increase as the number of N atoms in the ring increases.

DOI: 10.1002/slct.201800809

Synthesis, Structure and Anion Sensing Properties of a Dicationic Bis(imidazolium)‐Based Cyclophane

The preparation and sensing properties of a tetracyclic cyclophane receptor containing two imidazolium rings as anion binding sites and two fluorene rings as fluorescent signaling units, are reported. The receptor behaves as a selective fluorescent chemosensor molecule for inorganic phosphates. 1H‐NMR spectroscopical data clearly indicate the simultaneous occurrence of charge‐assisted aromatic and aliphatic C−H noncovalent interactions. PCM/DFT calculations have been carried out to predict the structures of the complexes formed with one and two molecules of PF₆−, H₂PO₄− and HP₂O₇³−

DOI: 10.1021/acs.jpca.8b01551

Fostering the Basic Instinct of Boron in Boron–Beryllium Interactions

A set of complexes L2HB···BeX2 (L = CNH, CO, CS, N2, NH3, NCCH3, PH3, PF3, PMe3, OH2; X = H, F) containing a boron–beryllium bond is described at the M06-2X/6-311+G(3df,2pd)//M062-2X/6-31+G(d) level of theory. In this quite unusual bond, boron acts as a Lewis base and beryllium as a Lewis acid, reaching binding energies up to −283.3 kJ/mol ((H2O)2HB···BeF2). The stabilization of these complexes is possible thanks to the σ-donor role of the L ligands in the L2HB···BeX2 structures and the powerful acceptor nature of beryllium. According to the topology of the density, these B–Be interactions present positive laplacian values and negative energy densities, covering different degrees of electron sharing. ELF calculations allowed measuring the population in the interboundary B–Be region, which varies between 0.20 and 2.05 electrons upon switching from the weakest ((CS)2HB···BeH2) to the strongest complex ((H2O)2HB···BeF2). These B–Be interactions can be considered as beryllium bonds in most cases.

DOI: 10.1021/acs.jpca.8b00236

Hydrogen and Halogen Bonding in Cyclic FH_(4-n):FCl_n Complexes, for n = 0–4

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the six unique cyclic quaternary complexes FH:FH:FH:FH, FH:FH:FH:FCl, FH:FH:FCl:FCl, FH:FCl:FH:FCl, FH:FCl:FCl:FCl, and FCl:FCl:FCl:FCl stabilized by F–H···F hydrogen bonds and F–Cl···F halogen bonds. The binding energies of these complexes decrease as the number of FH molecules decreases, and therefore as the number of hydrogen bonds decreases, indicating that hydrogen bonds are primarily responsible for stabilities. Nonadditivities of binding energies are synergistic for complexes with 4, 3, and 2 FH molecules, but antagonistic for those with 1 and 0 FH molecules. In addition to depending on changes in F–F, F–H, and F–Cl distances, complex binding energies are also influenced by two sets of angular parameters. These include the external F–F–F angles which must sum to 360° in these cyclic structures, and the internal H–F–F angles for hydrogen bonds and F–Cl–F angles for halogen bonds, which measure the deviation from linearity of these bonds. Transition structures present the barriers to converting an equilibrium structure to an equivalent equilibrium structure on the potential surfaces. These barriers increase as the number of FH molecules decreases. EOM-CCSD spin–spin coupling constants 2hJ(F–F) across hydrogen bonds in complexes tend to increase with decreasing F–F distance. They increase dramatically in transition structures, but show no dependence on the F–F distance. The one-bond coupling constants 1hJ(F–H) are relatively small and negative in complexes, increase dramatically, and are positive in transition structures. 1J(F–H) values are greatest for the covalent F–H bond. Coupling constants 1xJ(F–Cl) across halogen bonds are relatively small and positive in complexes, and increase dramatically in transition structures. The largest values of 1J(F–Cl) are found for covalent bonds.

DOI: 10.1016/j.cplett.2018.01.039

Remote modulation of singlet–triplet gaps in carbenes

The modulation of the singlet-triplet (S/T) gap of phenyl-carbene derivatives by hydrogen bond formation has been studied using the G4(MP2) computational method. The complexation of the aromatic ring substituents (–NH₂, –OH, –PH₂, –SH) in meta- and para-positions with water and the protonation or deprotonation of such groups have a remarkable influence on the S/T gaps, reaching S/T gap variations from 25.7 to 93.7 kJ mol⁻¹. This variation is linearly related to the binding energy difference of the S/T configurations. Importantly, the triplet and singlet electronic configurations are systematically favored in the protonated and deprotonated forms, respectively, in all cases.

DOI: 10.1021/acs.jpca.7b11952

Hydrogen-Bonding Acceptor Character of Be3, the Beryllium Three-Membered Ring

The ability of Be₃ as a hydrogen bond acceptor has been explored by studying the potential complexes between this molecule and a set of hydrogen bond donors (HF, HCl, HNC, HCN, H₂O, and HCCH). The electronic structure calculations for these complexes were carried out at the MP2 and CCSD(T) computational levels together with an extensive NBO, ELF, AIM, and electrostatic potential characterization of the isolated Be₃ system. In all the complexes, the Be–Be σ bond acts as electron donor, with binding energies between 19 and 6 kJ mol^–1. A comparison with the analogous cyclopropane:HX complexes shows similar binding energies and contributions of the DFT-SAPT energetic terms. A blue-shift of the harmonic frequencies of Be₃ is observed upon complexation.

DOI: 10.1002/chem.201705047

Large Proton-Affinity Enhancements Triggered by Noncovalent Interactions

The proton affinity (PA) of the hydroxyl group of a series of compounds YH_xOH (x=0–3; Y=Li, Na, Be, Mg, B, Al, Si, P, S, Cl, H) always increases on interaction with Lewis bases (LB=NH₃, H₂CNH, HCN) according to electronic structure calculations at the G4 level of theory. The LB:YH_xOH complexes experience enhancements of PA from 24.8 to 304.8 kJ mol⁻¹ for the OH group with respect to the free compounds. This enhancement is related to the ability of these YH_xOH compounds, acting as Lewis acids, to form noncovalent interactions of different kinds. Interestingly, weakly interacting systems can give rise to PA enhancements equal to or even larger than those of strongly interacting systems, as observed in the halogen-bonded complex H₂CNH⋅⋅⋅ClOH, whereby the free compound (PA=642.4 kJ mol⁻¹) is transformed into a base (PA=947.2 kJ mol⁻¹) that is stronger than pyridine.

DOI: 10.1039/C7CP07891A

Alkaline-earth (Be, Mg and Ca) bonds at the origin of huge acidity enhancements

The interaction between alkaline-earth derivatives with the general formula X₂M (X = H, F and Cl; M = Be, Mg and Ca) and a set of Lewis bases, including first and second-row hydrides, namely YH_n (Y = O, N, F, S, P and Cl) hydrides, as well as other typical cyclic organic bases, such as aniline, 1H-1,2,3-triazole, 1H-tetrazole and phenylphosphine, was investigated using the G4 ab initio composite method. Contrary to what was expected, it was found that the interactions involving Mg and Ca derivatives were not necessarily weaker than those between beryllium bonds. The origin is two-fold: larger deformation of the interacting systems when Be-derivatives are involved and appearance of secondary non-covalent interactions in the formation of some of the Mg- and Ca-containing complexes. Hence, the dissociation of the latter complexes may require higher enthalpies than that of the Be complexes. These deformations are triggered by a significant redistribution of electron density of the two interacting moieties, which also result in dramatic changes in the reactivity of the interacting compounds and in particular in the intrinsic basicity of the Lewis bases investigated, to the point that conventional bases, such as ammonia or aniline, upon complexation with MCl₂ (M = Be, Mg and Ca), become stronger Brønsted acids than phosphoric acid, whereas other bases, such as 1H-tetrazole, become stronger acids than perchloric acid.

DOI: 10.1007/s00894-017-3551-1

Are beryllium-containing biphenyl derivatives efficient anion sponges?

The structures and stabilities of 2,2′-diBeX-1,1′-biphenyl (X = H, F, Cl, CN) derivatives and their affinities for F⁻, Cl⁻, and CN⁻ were theoretically investigated using a B3LYP/6–311 + G(3df,2p)//B3LYP/6–31 + G(d,p) model. The results obtained show that the 2,2′-diBeX-1,1′-biphenyl derivatives (X = H, F, Cl, CN) exhibit very high F⁻, Cl⁻, and CN⁻ affinities, albeit lower than those reported before for their 1,8-diBeX-naphthalene analogs, in spite of the fact that the biphenyl derivatives are more flexible than their naphthalene counterparts. Nevertheless, some of the biphenyl derivatives investigated are predicted to have anion affinities larger than those measured for SbF₅, which is considered one of the strongest anion capturers. Therefore, although weaker than their naphthalene analogs, the 2,2′-diBeX-1,1′-biphenyl derivatives can still be considered powerful anion sponges. This study supports the idea that compounds containing –BeX groups in chelating positions behave as anion sponges due to the electron-deficient nature and consequently high intrinsic Lewis acidity of these groups.

DOI: 10.1002/cphc.201700682

Simultaneous Occurrence of Quadruple Lewis Acid–Base Interactions between Selenium Atoms in Selenocarbonyl Dimers

High-level quantum chemical calculations are performed to investigate C=Se⋅⋅⋅Se=C interactions. Bounded structures are found with binding energies between −4 and −7 kJ mol⁻¹. An energy decomposition analysis shows that dispersion is the more attractive term, and in all cases save one, the electrostatic interaction is attractive despite each selenium atom having a positive σ-hole at the extension of the C=Se bond. The topological analysis of the molecular electrostatic potential and L(r)=−∇²ρ(r) function, and natural bond orbital analysis reveal that these particular Se⋅⋅⋅Se contacts can be considered to be quadruple Lewis acid–base interactions.

DOI: 10.1002/cphc.201700819

Hydrogen Bond versus Halogen Bond in Cation–Cation Complexes: Effect of the Solvent

Competition between hydrogen- (HB) and halogen-bonded (XB) 4-ammoniumpyridine and halogenammonium (NH_nF_3−nX⁺; n=0–3; X=F, Cl, Br, and I) cation–cation complexes are explored by means of DFT calculations. HB and XB minima structures are found for all systems in the gas phase. As the number of fluorine atoms increases, the HB complexes are more favored than those of XB. Proton transfer is generally observed in complexes with two, three, or four halogen atoms. The XB complexes evolve from traditional halogen bonds, to halogen-shared complexes, and to ionic complexes as the number of fluorine atoms increases. The dissociation transition states and their corresponding barriers are also characterized; the barriers increase as the number of fluorine atoms increases. The results if solvent effects are considered indicate that, even in an apolar solvent, such as n-hexane, most of the complexes have favorable binding energies. Atoms-in-molecules theory is used to analyze the complexes, and results in good correlations between electron density and total electron energy density (Η) values with the intermolecular bond length. According to the Η values obtained, the covalency of these interactions starts to manifest at distances around 72–74 % the sum of the van der Waals radii of the interacting atoms.

DOI: 10.1021/acs.jpca.7b09678

Enhancing Intramolecular Chalcogen Interactions in 1-Hydroxy-8-YH-naphthalene Derivatives

Forty-two peri-substituted naphthalene derivatives presenting chalcogen weak interactions were studied. They correspond to O···Y interactions, Y being O, S, and Se. While the O atom bears H or CH₃ substituents (OH and OCH₃ groups), the Y atom is substituted by H, F, and CN to explore the effect of these electron-donating and electron-withdrawing substituents on the chalcogen bond strength. The effect of F and CH₃ substituents on positions ortho/para (2,4,5,7 of the naphthalene ring) was also studied. Optimizations were performed at the MP2/aug-cc-pVDZ, and binding energies were performed at the MP2/aug-cc-pVDZ followed by an MP2/CBS estimation. The main properties studied were geometries, energies (E_b, E_iso, and E_def), the molecular electrostatic potential, electron density shifts, natural bond order E(2) energies, and the relationship between these properties.

DOI: 10.3390/molecules22111955  (OpenAccess)

Halogen Bonding Involving CO and CS with Carbon as the Electron Donor

MP2/aug’-cc-pVTZ calculations have been carried out to investigate the halogen-bonded complexes formed when CO and CS act as electron-pair donors through C to ClF, ClNC, ClCl, ClOH, ClCN, ClCCH, and ClNH₂. CO forms only complexes stabilized by traditional halogen bonds, and all ClY molecules form traditional halogen-bonded complexes with SC, except ClF which forms only an ion-pair complex. Ion-pair complexes are also found on the SC:ClNC and SC:ClCl surfaces. SC:ClY complexes stabilized by traditional halogen bonds have greater binding energies than the corresponding OC:ClY complexes. The largest binding energies are found for the ion-pair SC–Cl⁺:⁻Y complexes. The transition structures which connect the complex and the ion pair on SC:ClNC and SC:ClCl potential surfaces provide the barriers for inter-converting these structures. Charge-transfer from the lone pair on C to the σ-hole on Cl is the primary charge-transfer interaction stabilizing OC:ClY and SC:ClY complexes with traditional halogen bonds. A secondary charge-transfer occurs from the lone pairs on Cl to the in-plane and out-of-plane π antibonding orbitals of ClY. This secondary interaction assumes increased importance in the SC:ClNH₂ complex, and is a factor leading to its unusual structure. C–O and C–S stretching frequencies and ¹³C chemical shieldings increase upon complex formation with ClY molecules. These two spectroscopic properties clearly differentiate between SC:ClY complexes and SC–Cl⁺:⁻Y ion pairs. Spin–spin coupling constants ^1xJ(C–Cl) for OC:ClY complexes increase with decreasing distance. As a function of the C–Cl distance, ^1xJ(C–Cl) and ¹J(C–Cl) provide a fingerprint of the evolution of the halogen bond from a traditional halogen bond in the complexes, to a chlorine-shared halogen bond in the transition structures, to a covalent bond in the ion pairs.

DOI: 10.1021/acs.jpca.7b08393 (open access)

Carbon–Carbon Bonding between Nitrogen Heterocyclic Carbenes and CO₂

Ab initio MP2/aug′-cc-pVTZ calculations were performed to identify equilibrium complexes and molecules and the transition structures that interconvert them, on the potential energy surfaces of a series of seven binary systems that have nitrogen heterocyclic carbenes (NHCs) as the electron-pair donors to CO2. Seven of the NHCs form complexes stabilized by C···C tetrel bonds, and six of these seven are also stabilized by a secondary interaction between an O of CO2 and the adjacent N–H group of the carbene. Six of the seven NHCs also form stable molecules with C–C covalent bonds, and with one exception, these molecules have binding energies that are significantly greater than the binding energies of the complexes. Charge-transfer stabilizes all of the NHC:CO2 complexes and occurs from the C lone pair of the carbene to the CO2 molecule. The six complexes that have secondary stabilizing interactions are also stabilized by back-donation of charge from the O to the adjacent N–H group of the carbene. Transition structures present barriers to the interconversion of complexes and molecules. With one exception, the barrier for converting a molecule to a complex is much greater than the barrier for the reverse reaction. Atoms in Molecules bonding parameters, shifts of IR C–O stretching and O–C–O bending frequencies, changes in NMR 13C chemical shieldings, and changes in C–C and C–O coupling constants as 1tJ(C–C) and J(C–O) for complexes and transition structures become 1J(C–C) and 2J(C–O) for molecules, are all consistent with the changing nature of the C···C tetrel bond in the complex through the transition state to a covalent C–C bond in the molecule.

DOI: 10.3390/molecules22101786  (Open Access)

Nucleophilicities of Lewis Bases B and Electrophilicities of Lewis Acids A Determined from the Dissociation Energies of Complexes B···A Involving Hydrogen Bonds, Tetrel Bonds, Pnictogen Bonds, Chalcogen Bonds and Halogen Bonds

It is shown that the dissociation energy De for the process B⋯A = B + A for 250 complexes B⋯A composed of 11 Lewis bases B (N₂, CO, HC≡CH, CH₂=CH₂, C₃H₆, PH₃, H₂S, HCN, H₂O, H₂CO and NH₃) and 23 Lewis acids (HF, HCl, HBr, HC≡CH, HCN, H₂O, F₂, Cl₂, Br₂, ClF, BrCl, H₃SiF, H₃GeF, F₂CO, CO₂, N₂O, NO₂F, PH₂F, AsH₂F, SO₂, SeO₂, SF₂, and SeF₂) can be represented to good approximation by means of the equation De=c′NBEA , in which NB is a numerical nucleophilicity assigned to B, EA is a numerical electrophilicity assigned to A, and c′ is a constant, conveniently chosen to have the value 1.00 kJ mol⁻¹ here. The 250 complexes were chosen to cover a wide range of non-covalent interaction types, namely: (1) the hydrogen bond; (2) the halogen bond; (3) the tetrel bond; (4) the pnictogen bond; and (5) the chalcogen bond. Since there is no evidence that one group of non-covalent interaction was fitted any better than the others, it appears the equation is equally valid for all the interactions considered and that the values of NB and EA so determined define properties of the individual molecules. The values of NB and EA can be used to predict the dissociation energies of a wide range of binary complexes B⋯A with reasonable accuracy

DOI: 10.1021/acs.jpca.7b08505  (OpenAccess)

Azines as Electron-Pair Donors to CO2 for N···C Tetrel Bonds

Ab initio MP2/aug′-cc-pVTZ calculations were performed to investigate tetrel-bonded complexes formed between CO2 and the aromatic bases pyridine, the diazines, triazines, tetrazines, and pentazine. Of the 23 unique equilibrium azine:CO2 complexes, 14 have planar structures in which a single nitrogen atom is an electron-pair donor to the carbon of the CO2 molecule, and 9 have perpendicular structures in which two adjacent nitrogen atoms donate electrons to CO2, with bond formation occurring along an N–N bond. The binding energies of these complexes vary from 13 to 20 kJ mol–1 and decrease as the number of nitrogen atoms in the ring increases. For a given base, planar structures have larger binding energies than perpendicular structures. The binding energies of the planar complexes also tend to increase as the distance across the tetrel bond decreases. Charge transfer in the planar pyridine:CO2 complex occurs from the N lone pair to a virtual nonbonding orbital of the CO2 carbon atom. In the remaining planar complexes, charge transfer occurs from an N lone pair to the remote in-plane π*C–O orbital. In perpendicular complexes, charge transfer occurs from an N–N bond to the adjacent π*O–C–O orbital of CO2. Decreases in the bending frequency of the CO2 molecule and in the 13C chemical shielding of the C atom of CO2 upon complex formation are larger in planar structures compared to perpendicular structures. EOM-CCSD spin–spin coupling constants 1tJ(N–C) for complexes with planar structures are very small but still correlate with the N–C distance across the tetrel bond.

DOI: 10.1039/C7FD00048K

Using one halogen bond to change the nature of a second bond in ternary complexes with P⋯Cl and F⋯Cl halogen bonds

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to determine the effect of the presence of one halogen bond on the nature of the other in ternary complexes H2XP:ClF:ClH and H2XP:ClF:ClF, for X = F, Cl, H, NC, and CN. The P⋯Cl bonds remain chlorine-shared halogen bonds in the ternary complexes H2XP:ClF:ClH, although the degree of chlorine sharing increases relative to the corresponding binary complexes. The F⋯Cl bonds in the ternary complexes remain traditional halogen bonds. The binding energies of the complexes H2XP:ClF:ClH increase relative to the corresponding binary complexes, and nonadditivities of binding energies are synergistic. In contrast, the presence of two halogen bonds in the ternary complexes H2XP:ClF:ClF has a dramatic effect on the nature of these bonds in the four most strongly bound complexes. In these, chlorine transfer occurs across the P⋯Cl halogen bond to produce complexes represented as (H2XP–Cl)+:−(F:ClF). In the ion-pair, the cation is also halogen bonded to the anion by a Cl⋯F− halogen bond, while the anion is stabilized by an −F⋯Cl halogen bond. The central ClF molecule no longer exists as a molecule. The binding energies of the ternary H2XP:ClF:ClF complexes are significantly greater than the binding energies of the H2XP:ClF:ClH complexes, and nonadditivities exhibit large synergistic effects. The Wiberg bond indexes for the complexes H2XP:ClF, H2XP:ClF:ClH, and H2XP:ClF:ClF, and the cations (H2XP–Cl)+ reflect the changes in the P–Cl and Cl–F bonds. Similarly, EOM-CCSD spin–spin coupling constants are also consistent with the changes in these same bonds. In particular, 1xJ(P–Cl) in H2XP:ClF complexes becomes 1J(P–Cl) in the ternary complexes with chlorine-transferred halogen bonds. A plot of these coupling constants shows a change in the curvature of the trendline as chlorine-shared halogen bonds in H2XP:ClF:ClH become chlorine-transferred halogen bonds in H2XP:ClF:ClF. 1xJ(F–Cl) coupling constants also reflect changes in the nature of F⋯Cl halogen bonds.

DOI: 10.1002/poc.3690

Is it possible to use the 31P chemical shifts of phosphines to measure hydrogen bond acidities (HBA)? A comparative study with the use of the 15N chemical shifts of amines for measuring HBA

The geometries, energies, and nuclear magnetic resonance (NMR) chemical shifts of 3 bases (trimethylphosphine, trimethylamine, and trimethylphosphine oxide), their 3 protonated cations, and 15 hydrogen-bonded complexes (corresponding to the HF, HNC, HCN, HCCH, H2O, and CH3OH Brønsted acids) have been calculated at the B3LYP/6-311++G(d,p) level. The determination of hydrogen bond acidities by NMR is classically performed using the 31P chemical shifts Me3PO. This method is more reliable than the use of the 15N NMR chemical shifts of Me3N. This work shows that the 31P NMR chemical shifts of Me3P cannot be used. The raison of the difference between Me3P on one hand and Me3PO and Me3N on the other will be discussed.

DOI: 10.1021/acs.jpca.7b07886

Modulating the Proton Affinity of Silanol and Siloxane Derivatives by Tetrel Bonds

The proton affinity (PA) on the oxygen atom in silanol and siloxane derivatives is enhanced by the formation of tetrel bonds with small Lewis bases [B···R3SiOH, B···R3SiOSiR3, B···R3SiOSiR3···B; B = H2O, CO, NH3, HCN, H2S; R = H, Me], as shown by MP2/jul-cc-pVTZ calculations. The complexed systems become more basic than ether and other carbon-related compounds, and even more basic than pyridine in some specific cases, reaching values up to 959.4 kJ/mol (H3N···H3SiOSiH3···NH3 complex). Changes on PAs are directly related to very large binding energies for the protonated species. Topological methods and the natural bond orbital scheme are used to rationalize the observed trends. The PA enhancement should be taken into account when dealing with silanols and siloxanes in different environments.

DOI: 10.1007/s11224-017-0912-4

Borylene as an electron-pair donor for P…B pnicogen bonds

Ab initio MP2/aug’-cc-pVTZ calculations have been performed on the complexes (CO)2(HB):PXH2 and (N2)2(HB):PXH2, for X = F, Cl, NC, OH, CN, CCH, CH3, and H, in order to investigate the properties of these complexes which are stabilized by P…B pnicogen bonds, with B the electron-pair donor. The binding energies of these complexes exhibit an exponential dependence on the P-B distance, but they do not correlate with the MEP minima for (CO)2(HB) and (N2)2(HB), nor with the MEP maxima for PXH2. For fixed X, the binding energy of (N2)2(HB):PXH2 is greater than that of (CO)2(HB):PXH2. Charge-transfer stabilizes both series of complexes, and occurs from the B electron pair to the antibonding P-A σ orbital, with A the atom of X directly bonded to P. These charge-transfer energies also exhibit an exponential dependence on the P-B distance. In the complexes (CO)2(HB):PXH2, there is a second charge-transfer interaction from the lone pair on P to the antibonding π orbitals of the two C-O groups. Electron density analyses indicate that the P…B bonds in these complexes are stabilized by relatively weak interactions with little covalent character. The chemical shieldings of 11B are essentially unaffected by complex formation. In contrast, the shieldings of 31P increase from 10 to 50 ppm in the four most strongly bound complexes, but decrease by −4 to −12 ppm in the remaining complexes. For each series of complexes, EOM-CCSD spin-spin coupling constants 1pJ(P-B) increase quadratically with decreasing P-B distance. For fixed X, 1pJ(P-B) is greater for (CO)2(HB):PXH2 compared to (N2)2(HB):PXH2.

DOI: 10.1039/c7cp03664g

Beryllium-based fluorenes as efficient anion sponges

 The F−, Cl−, CN−, NO2−, NO3−, and SO42− anion affinities of 4,5-bis(BeX)-fluorene (X = H, F, Cl, CN, NC, and OCH3) derivatives have been calculated at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d,p) level of theory. The reliability of this approach was assessed using, for some suitable cases, the accurate G4MP2 ab initio composite method as a reference. The values obtained indicate that these derivatives exhibit anion affinities which are among the largest ones reported for single neutral molecules, and therefore these compounds behave as anion sponges, very much as their 1,8-diBeX-naphthalene analogues. This finding seems to confirm that both molecular frameworks, when adequately substituted at positions 1,8 in the case of naphthalene and 4,5 in the case of fluorene, may behave either as proton sponges, when the substituents are good electron donors, such as alkylamino groups, or as anion sponges, when the substituents are BeX groups, which are excellent electron acceptors. The behavior of these compounds in aqueous solution was also investigated. The interaction with water decreases the anion affinities, but still they are very large. More importantly the trends observed do not differ significantly from those found in the gas phase, in particular when monoanions are considered.

DOI: 10.1039/C7CP03661B

Modulation of in:out and out:out conformations in [X.X′.X′′] phosphatranes by Lewis acids

A theoretical study of [X.X′.X′′]phosphatrane:Lewis acid complexes has been carried out in order to analyze how the in:out and out:out conformations can be modulated by the interaction with Lewis acids (LA). It has been found that in:out structures are more stable in larger systems i.e. in [4.4.3]:LA and [4.4.4]:LA than in [3.3.3]:LA and [4.3.3]:LA. The results obtained for the relative energies in conjunction with electron density properties showed that upon complexation, in:out conformers become more stable with the increasing acidity of the corresponding Lewis acid. In fact, the binding energies found for in:out complexes are larger than those obtained for out:out complexes. The complexes with the largest relative energy favoring the in:out structure correspond to those with charged Lewis acids, followed by the complexes with ClF. In all cases, the complexes are cooperative, reaching a maximum value of 168.5 kJ mol−1 for the [4.3.3]:F+ complex.

DOI: 10.1016/j.cplett.2017.07.051

Halogen bonding with carbene bases

Ab initio MP2/aug’-cc-pVTZ calculations have been performed to determine the structures and binding energies of complexes formed from five singlet carbene bases acting as electron-pair donors to four ClX acids. Complexes with ClCCH and ClCN are stabilized by traditional halogen bonds. With one exception, complexes with ClNC and ClF are stabilized by (carbene-Cl)+⋯–NC and (carbene-Cl)+⋯–F ion-pair halogen bonds subsequent to chlorine transfer. These complexes have been characterized in terms of their structures, binding energies, charge-transfer energies, bonding properties, and spin-spin coupling constants.

DOI: 10.1021/acs.jpca.7b05220

The Curious Case of 2‑Propyl‑1H‑benzimidazole in the Solid State: An Experimental and Theoretical Study

2-Propyl-1H-benzimidazole (2PrBzIm) is a small molecule, commercially available, which displays a curious behavior in the solid state. 2PrBzIm, although devoid of chirality by fast rotation about a single bond of the propyl group in solution, crystallizes as a conglomerate showing chiroptical properties. An exhaustive analysis of its crystal structure and a wide range of experiments monitored by vibrational circular dichroism spectroscopy eliminated all possibilities of an artifact. What remains is a new example of the unexplained phenomenon of persistent supramolecular chirality.

DOI: 10.1002/chem.201701444

Trapping CO₂ by Adduct Formation with Nitrogen Heterocyclic Carbenes (NHCs): A Theoretical Study

Carbon dioxide can form compounds with nitrogen heterocyclic carbenes (NHCs) based on azoles through noncovalent interactions or by covalent bonding. A narrow dependence on the carbene structure has been observed for the preference for one or the other type of bonding, as revealed by a series of physicochemical descriptors. In our survey, a set of NHCs based on the azole family (three classical, three abnormal, and one remote) was shown to bind CO2 at the accurate G4MP2 computational level. In most cases, exothermic reaction profiles towards the covalently bound form were found, which reached stabilization enthalpies of up to −77 kJ mol⁻¹ for the remote carbene case. Both noncovalent and covalent minima and the corresponding transition state that connects them have been identified as stationary points along the reaction coordinate.

OpenAcess

Activation of Dinitrogen as A Dipolarophile in 1,3-Dipolar Cycloadditions: A Theoretical Study Using Nitrile Imines as “Octet” 1,3-Dipoles

Theoretical calculations at the G4MP2 level of theory demonstrate that it is possible to activate dinitrogen to make it react in dipolar cycloadditions using neutral beryllium derivatives and other neutral metallic compounds. For the particular case of beryllium, the barrier decreases more than 40 kJ·mol–1 with respect to the non-catalysed reaction. The activation achieved is lower than using diazonium salts (models of protonated N2), but still in a range that can be experimentally attainable.

DOI: 10.1016/j.molstruc.2017.06.101

Nitroxide stable radicals interacting as Lewis bases in hydrogen bonds: A search in the Cambridge structural data base for intermolecular contacts

1125 X-ray structures of nitroxide free radicals presenting intermolecular hydrogen bonds have been reported in the Cambridge Structural Database. We will report in this paper a qualitative and quantitative analysis of these bonds. The observation in some plots of an excluded region was statistically analyzed using convex hull and kernel smooting methodologies. A theoretical study at the MP2 level with different basis has been carried out indicating that the nitronyl nitroxide radicals (five electrons) lie just in between nitroso compounds (four electrons) and amine N-oxides (six electrons) as far as hydrogen-bond basicity is concerned.

DOI: 10.1002/cphc.201700187

Carbenes as Electron-Pair Donors for P⋅⋅⋅C Pnicogen Bonds

Ab initio MP2/aug′-cc-pVTZ calculations were performed on the P⋅⋅⋅C pnicogen-bonded complexes of the singlet carbene molecules C(NH2)2, C(OH)2, and cyclic C(OCH)2 [OHC] with H2XP molecules, with X=F, Cl, NC, OH, CH3, CN, CCH, and H. The H2XP:C(NH2)2 and H2XP:C(OH)2complexes have Cs symmetry and two different structures: one in which the symmetry plane of the complex and the local symmetry plane of the carbene are non-coplanar, and the other in which they are coplanar. The non-coplanar H2XP:C(NH2)2 and H2XP:C(OH)2 complexes arise only when X is one of the more electronegative substituents. Coplanar H2XP:C(NH2)2 complexes form when X is one of the more electropositive substituents, whereas coplanar H2XP:C(OH)2 complexes exist for all X. H2XP:C(NH2)2 and H2XP:C(OH)2 are stabilized by covalent P−C bonds or P⋅⋅⋅C pnicogen bonds, but co-planar H2(CH3)P:C(OH)2 and H3P:C(OH)2 are stabilized by O−H⋅⋅⋅P hydrogen bonds. The H2XP:OHC complexes have non-coplanar structures that are also stabilized by P−C covalent bonds or pnicogen bonds. The H2(CH3)P:OHC and H3P:OHC complexes in which the symmetry plane of the complex and the local symmetry plane of the carbene are perpendicular are stabilized by P⋅⋅⋅π bonds with P acting as the electron-pair donor to the OHC π system. The H2XP:C(NH2)2, H2XP:C(OH)2, and H2XP:OHC complexes are described in terms of their binding energies, charge-transfer energies, electron density properties, and equation-of-motion coupled cluster singles and doubles spin–spin coupling constants.

DOI: 10.1021/acs.jpca.7b03405 (OpenAccess)

Carbenes as Electron-Pair Donors to CO2 for C···C Tetrel Bonds and C−C Covalent Bonds

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to identify stable complexes and molecules and the transition structures that interconvert them on the potential surfaces of ten singlet carbene bases acting as electron-pair donors to CO2. The carbene bases include cyclic C(NHCH)2 or NHC, C(NH2)2, an oxygen heterocyclic carbene C(OCH)2 or OHC, C(OH)2, C(CH3)2, cyclic C3H2, CCCH2, CCl2, CCH2, and CF2. Carbene:CO2 complexes stabilized by C···C tetrel bonds have been found on all potential surfaces, whereas carbene–CO2 molecules stabilized by C–C covalent bonds have been found on eight surfaces. Three of these molecules have open structures with C2v symmetry, whereas the remaining have cyclic three membered C–O–C rings with Cs symmetry. The transition structures which connect the complex and the molecule are bound on three of the potential surfaces. Whether the transition structure is bound or unbound relative to the carbene and CO2 depends on the relationship among C–C distances at the three stationary points on the surface. Charge-transfer interactions stabilize carbene:CO2complexes. The primary charge transfer in complexes arises from electron donation from the carbene lone-pair to the CO2 molecule. There is also back-donation of charge from CO2 to the carbene in three complexes. Systematic changes in bonding properties occur as complexes go through transition structures and become molecules. EOM-CCSD inter- and intramolecular C–C and C–O spin–spin coupling constants have been computed and compared for complexes and molecules. A search of the CSD database found the (NH2)2C–CO2 structure and 17 NHC–CO2derivatives. Computed bond distances and angles have been compared with experimental data.

DOI:  10.1016/j.cplett.2017.02.012

Hydrogen-bonded complexes with carbenes as electron-pair donors

Ab initio MP2/aug′-cc-pVTZ calculations have been performed to investigate X-H⋯C hydrogen-bonded complexes involving eleven different carbene molecules as electron-pair donors and the acids FH, CNH, and NCH. Binding energies and charge-transfer energies increase as the X-C distances decrease, and exhibit an exponential dependence on these distances. EOM-CCSD spin-spin coupling constants ^2hJ(X-C) increase quadratically in absolute value as the corresponding X-C distance decreases. ¹J(F-H) and ¹J(N-H) decrease as the F-H and N-H distances, respectively, increase, but ¹J(C-H) shows little dependence on distance. All of these carbene complexes are stabilized by traditional hydrogen bonds.

DOI: 10.1007/s00214-017-2069-z

A theoretical study of the H_nF_4−nSi:N‑base (n = 1–4) tetrel‑bonded complexes

Tetrel-bonded complexes of HnF4−nSi with a N-base for n = 0–4 were explored by MP2 calculations. Configurations with H–Si···N and F–Si···N linear or nearly linear alignment in complexes were considered. Nine sp3 hybridized nitrogen bases NH3, NH2Cl, NH2F, NHCl2, NCl3, NFCl2, NHF2, NF2Cl, NF3 and nine sp ones NCNH2, NCCH3, NCOH, NP, NCCl, NCH, NCF, NCCN, N2 have been studied. It is shown that binding energies of the complexes depend strongly on the nature of the base involved in the complex. Complexes with NH3 bases present the highest binding energies. In the stronger complexes, the silicon molecules suffer important geometrical distortions. NBO and AIM methodologies have been applied in order to describe properly the intermolecular Si···N contact. F atoms in equatorial position at silicon acid provoke a deviation from linearity of the Si···N electron density bond path trajectory.

DOI: 10.1021/acs.jpca.6b12553

Lone-Pair Hole on P: P···N Pnicogen Bonds Assisted by Halogen Bonds

Ab initio MP2/aug’-cc-pVTZ calculations have been performed on the binary complexes XY:PH3for XY = ClCl, FCl, and FBr; and PH3:N-base for N-base = NCH, NH3, NCF, NCCN, and N2; and the corresponding ternary complexes XY:PH3:N-base, to investigate P···N pnicogen bond formation through the lone-pair hole at P in the binary complexes and P···N pnicogen-bond formation assisted by P···Y halogen bond formation through the σ-hole at Y. Although the binary complexes PH3:N-base that form through the lone-pair hole have very small binding energies, they are not equilibrium structures on their potential surfaces. The presence of the P···Y halogen bond makes PH3 a better electron-pair acceptor through its lone-pair hole, leading to stable ternary complexes XY:PH3:N-base. The halogen bonds in ClCl:PH3 and ClCl:PH3:NCCN are traditional halogen bonds, but in the remaining binary and ternary complexes, they are chlorine- or bromine-shared halogen bonds. For a given nitrogen base, the P···N pnicogen bond in the ternary complex FCl:PH3:N-base appears to be stronger than that bond in FBr:PH3:N-base, which is stronger than the P···N bond in the corresponding ClCl:PH3:N-base complex. EOM-CCSD spin–spin coupling constants for the binary and ternary complexes with ClCl and FCl are also consistent with the changing nature of the halogen bonds in these complexes. At long P–Cl distances, the coupling constant 1xJ(P–Cl) increases with decreasing distance but then decreases as the P–Cl distance continues to decrease, and the halogen bonds become chlorine-shared bonds. At the shorter distances, 1xJ(P–Cl) approaches the value of 1J(P–Cl) for the cation +(Cl–PH3). The coupling constants 1pJ(P–N) are small and, with one exception, are greater in ClCl:PH3:N-base complexes compared to that in FCl:PH3:N-base, despite the shorter P–N distances in the latter.

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Theoretical Study of Intramolecular Interactions in Peri-Substituted Naphthalenes: Chalcogen and Hydrogen Bonds

A theoretical study of the peri interactions, both intramolecular hydrogen (HB) and chalcogen bonds (YB), in 1-hydroxy-8YH-naphthalene, 1,4-dihydroxy-5,8-di-YH-naphthalene, and 1,5-dihydroxy-4,8-di-YH-naphthalene, with Y = O, S, and Se was carried out. The systems with a OH:Y hydrogen bond are the most stable ones followed by those with a chalcogen O:Y interaction, those with a YH:O hydrogen bond (Y = S and Se) being the least stable ones. The electron density values at the hydrogen bond critical points indicate that they have partial covalent character. Natural Bond Orbital (NBO) analysis shows stabilization due to the charge transfer between lone pair orbitals towards empty Y-H that correlate with the interatomic distances. The electron density shift maps and non-covalent indexes in the different systems are consistent with the relative strength of the interactions. The structures found on the CSD were used to compare the experimental and calculated results.

DOI: 10.1039/C6CP04940K

Supramolecular organization of perfluorinated 1H-indazoles in the solid state using X-ray crystallography, SSNMR and sensitive (VCD) and non sensitive (MIR, FIR and Raman) to chirality vibrational spectroscopies

1H-Indazole derivatives exhibit a remarkable property since some of them form chiral supramolecular structures starting from achiral monomers. The present work deals with the study of three perfluorinated 1H-indazoles that resolve spontaneously as conglomerates. These conglomerates can contain either a pure enantiomer (one helix) or a mixture of both enantiomers (both helices) with an enantiomeric excess (e.e.) of one of them. The difficulty of the structural analysis of these types of compounds is thus clear. We outline a complete strategy to determine the structures and configurations (M or P helices) of the enantiomers (helices) forming the conglomerates of these perfluorinated 1H-indazoles based on X-ray crystallography, solid state NMR spectroscopy and different solid state vibrational spectroscopies that are either sensitive (VCD) or not (FarIR, IR and Raman) to chirality, together with quantum chemical calculations (DFT).

DOI: 10.1002/chem.201604325

Beryllium-Based Anion Sponges: Close Relatives of Proton Sponges

Through the use of high-level ab initio and density functional calculations it is shown that 1,8-diBeX-naphthalene (X=H, F, Cl, CN, CF₃, C(CF₃)₃) derivatives behave as anion sponges, very much as 1,8-bis(dimethylamino)naphthalene derivatives behave as proton sponges. The electron-deficient nature of the BeX substituents, which favors strong charge transfer from the anion towards the former, results in anion affinities that are among the largest ones reported for single neutral molecules.

DOI: 10.1039/c6cp06474d

Unusual acid–base properties of the P4 molecule in hydrogen-, halogen-, and pnicogen-bonded complexes

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate hydrogen bonding, halogen bonding, and pnicogen bonding involving tetrahedral P₄ and the FH, ClH, and FCl molecules. P₄ has three unique interaction sites: at a vertex (designated the P₁ atom); at an edge (the P₂–P₃ bond); and at the P₂–P₃–P₄ face. The uniqueness of molecular P₄ is its ability to act as an electron donor and an electron acceptor at the same site, except for the P₂–P₃ bond, which is only an electron donor. FCl and FH form five different complexes with P₄, but ClH forms only three. The type of complex formed and its binding energy depend on both the interaction site of molecular P₄ and the interacting molecule. For all complexes with FH, ClH, and FCl, the binding energies at a given site with the P₄ molecule acting as the base are greater than the binding energies when P₄ is the acid. Thus, P₄ is a better electron donor than an electron acceptor. Charge-transfer interactions and EOM-CCSD spin–spin coupling constants across hydrogen, halogen, and pnicogen bonds are reported for all of the P₄ complexes. Relative to ¹J(P_i–P_j) in molecular P₄, ¹J(P₁–P₂) coupling constants decrease in absolute value and ¹J(P₂–P₃) coupling constants increase in pnicogen-bonded complexes and the complex with FCl that has a P⋯F halogen bond. Absolute values of ¹J(P₁–P₂) increase and those of ¹J(P₂–P₃) decrease in hydrogen-bonded complexes and complexes with P⋯Cl halogen bonds. ¹J(P₁–P₂) and ¹J(P₂–P₃) exhibit a single linear correlation with the corresponding P_i–P_j distances.

DOI: 10.1039/C6NJ01334A

Weak interactions within nitryl halide heterodimers

A theoretical study of nitryl halide heterodimers has been carried out using SCS-RI-MP2 and CCSD(T) at the complete basis set (CBS) calculations. For this purpose, 66 heterodimers have been characterized as minima and arranged in six groups depending on the interactions involved and their geometrical arrangements. The CCSD(T)/CBS interaction energies vary between −0.6 and −11.1 kJ mol⁻¹. The heavier the halogen atoms, the larger the interaction energies. Natural bond orbital (NBO) and “atoms-in-molecules” (AIM) theories were then used to analyze the complexes, confirming the presence of halogen, chalcogen, and π-hole interaction bonds. The largest charge-transfer energy contributions were found for halogen bonded complexes (up to 29.1 kJ mol⁻¹). Furthermore, the physical nature of the interactions was studied using symmetry-adapted perturbation theory (SAPT) calculations, and it was concluded that dispersion was the major source of attraction, although electrostatics is important in halogen bonded complexes.

DOI: 10.1039/C6CP03662G

Cation–cation and anion–anion complexes stabilized by halogen bonds

Stable minima showing halogen bonds between charged molecules with the same sign have been explored by means of theoretical calculations. The dissociation transition states and their corresponding barriers have also been characterized. In all cases, the results indicate that the complexes are thermodynamically unstable but kinetically stable with respect to the isolated monomers in gas phase. A corrected binding energy profile by removing the charge–charge repulsion of the monomers shows a profile similar to the one observed for the dissociation of analogous neutral systems. The nature of the interaction in the minima and TSs has been analyzed using the symmetry adapted perturbation theory (SAPT) method. The results indicate the presence of local favorable electrostatic interactions in the minima that vanish in the TSs. Natural bond orbital (NBO) and “atoms-in-molecules” (AIM) theories were used to analyze the complexes, obtaining good correlations between Laplacian and electron density values with both bond distances and charge-transfer energy contributions E(2). The largest E(2) orbital interaction energies for cation–cation and anion–anion complexes are 561.2 and 197.9 kJ mol⁻¹, respectively.

DOI: 10.1002/cphc.201600435

Boron as an Electron-Pair Donor for B···Cl Halogen Bonds

MP2/aug’-cc-pVTZ calculations have been carried out to investigate boron as an electron-pair donor in halogen-bonded complexes (CO)₂(HB):ClX and (N₂)₂(HB):ClX, for X = F, Cl, OH, NC, CN, CCH, CH₃, and H.  Equilibrium halogen-bonded complexes with boron the electron-pair donor have been found on all of the potential surfaces except for (CO)₂(HB):ClCH₃ and (N₂)₂(HB):ClF.  The majority of these complexes are stabilized by traditional halogen bonds, except for (CO)₂(HB):ClF, (CO)₂(HB):ClCl, (N₂)₂(HB):ClCl, and (N₂)₂(HB):ClOH which are stabilized by chlorine-shared halogen bonds.  These complexes have increased binding energies and shorter B-Cl distances.  Charge transfer stabilizes all complexes, and occurs from the B lone pair to the σ* Cl-A orbital of ClX, with A the atom of X directly bonded to Cl.  A second reduced charge transfer interaction occurs in (CO)₂(HB):ClX complexes from the Cl lone pair to the π* C≡O orbitals.  EOM-CCSD spin-spin coupling constants ^1xJ(B-Cl) across the halogen bonds are also indicative of the changing nature of this bond.  ^1xJ(B-Cl) values for both series of complexes are positive at long distances, increase as the distance decreases, and then decrease as the halogen bonds change from traditional to chlorine-shared bonds, and begin to approach the values for the covalent bonds in the corresponding ions [(CO)₂(HB)-Cl]⁺ and [(N₂)₂(HB)-Cl]⁺.  Changes in ¹¹B chemical shieldings upon complexation correlate with changes in the charges on B. 

DOI: 10.1021/acs.joc.6b01146

Comparative Study of Charge-Assisted Hydrogen- and Halogen-Bonding Capabilities in Solution of Two-Armed Imidazolium Receptors toward Oxoanions

Two-armed imidazolium-based anion receptors have been prepared. The central 2,7-disubstituted naphthalene ring features two photoactive anthracene end-capped side arms with central 2-bromoimidazolium or hydrogen-bonding imidazolium receptors. Combined emission and 1H and 31P NMR studies carried out in the presence of a wide variety of anions reveal that only HP2O73–, H2PO4–, SO42–, and F– anions promoted noticeable changes. The halogen receptor 62+·2PF6– acts as a selective fluorescent molecular sensor for H2PO4– anions, since only this anion promotes the appearance of the anthracene excimer emission band, whereas it remains unchanged in the presence of the other tested anions. In addition this halogen receptor behaves as a chemodosimeter toward HP2O73– anion, through its transformation into the corresponding bis-imidazolone after debromination by the action of the basic anion. The association constant values of the halogen-bonding complexes in a competitive solvent CD3CN/MeOD (8/2) mixture with H2PO4– and SO42– anions are higher than those found for the hydrogen-bonding counterpart. In contrast, in the less competitive CH3CN solvent higher binding affinity for anions corresponds to the hydrogen-bonding receptor 72+·2PF6–. In addition, the receptor 62+·2PF6– represents a useful alternative as an imaging agent in living cells in a wide range of emission wavelengths.

DOI: 10.1021/acs.jpca.6b05367

Researchgate

B_⁴H₄ and B₄(CH₃)₄ as Unique Electron Donors in Hydrogen-Bonded and Halogen-Bonded Complexes

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out on B4H4 and B4(CH3)4 to investigate the base properties of these molecules with Td symmetry. Each face of the tetrahedral structure of B4H4 and B4(CH3)4 is stabilized by a two-electron, three-center B–B–B bond. The face uses these two electrons to act uniquely as an electron-pair donor for the formation of stable hydrogen-bonded and halogen-bonded complexes with C3v symmetry. The hydrogen-bonded complexes are B4H4:HY and B4(CH3)4:HY, with HY = HNC, HF, HCl, HCN, and HCCH; the halogen-bonded complexes are B4H4:ClY and B4(CH3)4:ClY, with ClY = ClF, ClCl, ClNC, ClCN, ClCCH, and ClH. The absolute values of the binding energies of the hydrogen-bonded complexes B4(CH3)4:HY and of the halogen-bonded complexes B4(CH3)4:ClY are significantly greater than the binding energies of the corresponding complexes with B4H4. The binding energies of each series correlate with the distance from the hydrogen-bonded H atom or halogen-bonded Cl atom to the centroid of the interacting face. Charge transfer stabilizes all complexes and occurs from the B2–B3–B4 orbital of the face to the antibonding H–X orbital of HY in hydrogen-bonded complexes and to the antibonding Cl–X orbital of ClY in halogen-bonded complexes, with X being the atom of Y that is directly bonded to either H or Cl. For fixed HY, EOM-CCSD spin–spin coupling constants J(X–B1) are greater than J(X–Bn) for complexes B4H4:HY, even though the X–B1 distances are longer. B1 and Bn are the atoms at the apex and in the interacting face, respectively. Similarly, for complexes B4H4:ClY, J(Cl–B1) is greater than J(Cl–Bn). In the halogen-bonded complexes, both coupling constants correlate with the corresponding distances.

DOI: 10.1002/anie.201603690

Exergonic and Spontaneous Production of Radicals through Beryllium Bonds

High-level ab initio calculations show that the formation of radicals, by the homolytic bond fission of Y−R (Y=F, OH, NH₂; R=CH₃, NH₂, OH, F, SiH₃, PH₂, SH, Cl, NO) bonds is dramatically favored by the association of the molecule with BeX₂ (X=H and Cl) derivatives. This finding is a consequence of two concomitant effects, the significant activation of the Y−R bond after the formation of the beryllium bond, and the huge stabilization of the F^. (OH^., NH₂^.) radical upon BeX₂ attachment. In those cases where R is an electronegative group, the formation of the radicals is not only exergonic, but spontaneous.

DOI: 10.1016/j.comptc.2016.06.020

Structure, binding energy and chiral discrimination in oxathiirane homodimers

Oxathiirane (XHCSO) homodimers bonded by hydrogen bonds (HB) and chalcogen bonds (YB) were studied at the Møller-Plesset (MP2) computational level. Binding energies obtained at the Coupled-Cluster level up to the Complete Basis Set limit [CCSD(T)/CBS] indicate that HB complexes present stronger binding modes than the YB complexes. In terms of chiral discrimination energy, R,S complexes are favored over R,Rcomplexes with the exceptions of SiCl₃ and SiF₃ derivatives. Natural Bond Orbital (NBO) results are in agreement with the interaction energies in the case of the HB complexes, but could not discriminate between R,R and R,S in the YB complexes. The lack of correlation between molecular electrostatic values on the 0.001 a.u. and binding energies, in addition to the discrepancies between Atoms in Molecules (AIM) and NBO results may suggest that the electrostatics is not the dominant term in the interaction energy. This was corroborated by the Localized Molecular Orbital Energy Decomposition Analysis (LMOEDA) calculations which showed that the exchange and dispersion terms are the most important attractive components for all the complexes studied, contributing up to 50.6% and 42.5% to the total attractive forces respectively.

DOI: 10.1002/chem.201600788

Charged versus Neutral Hydrogen-Bonded Complexes: Is There a Difference in the Nature of the Hydrogen Bonds?

A theoretical study on some carboxylic acid dimers formed by positively or negatively charged molecules has been carried out by using DFT methods. The resulting dimers possess either a charge of +2 or −2. In addition, the corresponding neutral complexes have also been considered. The electron density distribution described by the atoms in molecules and the natural bond orbital methods, as well as the electric field maps of the systems, have been analyzed and compared without finding significant differences between the neutral and ionic complexes. The interaction energy along the dissociation path of the charged dimers shows both a local minimum and a local maximum, defining a stability region between them. When this energetic profile is recalculated by removing the repulsion between the charged groups, it resembles to those of the neutral molecules. Hence, the characteristics of the charged dimers are similar to those of the neutral ones: the addition of a repulsion term for the charged groups permits to retrieve the energetic profiles dependence with the distance in the charged system. The interacting quantum atom (IQA) method has been used to calculate the interaction energy terms, including the classic Coulombic term between the whole molecules and the corresponding of the carboxylic acid groups. The IQA results show repulsive electrostatic interactions when the whole molecules are considered in the ionic complexes, but attractive ones between the carboxylic groups in both neutral and ionic complexes.

DOI: 10.1016/j.cplett.2016.05.030

ResearchGate

Anionic complexes of F-
and Cl-
with substituted methanes: Hydrogen, halogen, and tetrel bonds

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to investigate the anionic complexes X^–:CX(F_nH_3-n), for X = F, Cl, and n = 0 - 3. These complexes are stabilized by tetrel, hydrogen, and halogen bonds. Hydrogen-bonded complexes are the most stable complexes and halogen-bonded complexes are the least stable, with one exception. Charge-transfer across intermolecular bonds stabilizes all complexes, and occurs from the anion lone-pair to a σ^∗ orbital of the substituted methane. EOM-CCSD spin-spin coupling constants ^1tJ(X-C) across intermolecular tetrel bonds, ^2hJ(C-X) across hydrogen bonds, and ^1xJ(Cl-Cl) and ^2xJ(C-Cl) across halogen bonds correlate with intermolecular distances.

DOI: 10.1002/cphc.201600048

Researchgate

Using (FH)₂ and (FH)₃ to Bridge the σ-Hole and the Lone Pair at P in Complexes with H₂XP, for X=CH₃, OH, H, CCH, F, Cl, NC, and CN

Ab initio MP2/aug′-cc-pVTZ calculations are used to investigate the binary complexes H2XP:HF, the ternary complexes H2XP:(FH)2, and the quaternary complexes H2XP:(FH)3, for X=CH3, OH, H, CCH, F, Cl, NC, and CN. Hydrogen-bonded (HB) binary complexes are formed between all H2XP molecules and FH, but only H2FP, H2ClP, and H2(NC)P form pnicogen-bonded (ZB) complexes with FH. Ternary complexes with (FH)2 are stabilized by F−H⋅⋅⋅P and F−H⋅⋅⋅F hydrogen bonds and F⋅⋅⋅P pnicogen bonds, except for H2(CH3)P:(FH)2 and H3P:(FH)2, which do not have pnicogen bonds. All quaternary complexes H2XP:(FH)3 are stabilized by both F−H⋅⋅⋅P and F−H⋅⋅⋅F hydrogen bonds and P⋅⋅⋅F pnicogen bonds. Thus, (FH)2 with two exceptions, and (FH)3 can bridge the σ-hole and the lone pair at P in these complexes. The binding energies of H2XP:(FH)3complexes are significantly greater than the binding energies of H2XP:(FH)2 complexes, and nonadditivities are synergistic in both series. Charge transfer occurs across all intermolecular bonds from the lone-pair donor atom to an antibonding σ* orbital of the acceptor molecule, and stabilizes these complexes. Charge-transfer energies across the pnicogen bond correlate with the intermolecular P−F distance, while charge-transfer energies across F−H⋅⋅⋅P and F−H⋅⋅⋅F hydrogen bonds correlate with the distance between the lone-pair donor atom and the hydrogen-bonded H atom. In binary and quaternary complexes, charge transfer energies also correlate with the distance between the electron-donor atom and the hydrogen-bonded F atom. EOM-CCSD spin-spin coupling constants 2hJ(F–P) across F−H⋅⋅⋅P hydrogen bonds, and 1pJ(P–F) across pnicogen bonds in binary, ternary, and quaternary complexes exhibit strong correlations with the corresponding intermolecular distances. Hydrogen bonds are better transmitters of F–P coupling data than pnicogen bonds, despite the longer F⋅⋅⋅P distances in F−H⋅⋅⋅P hydrogen bonds compared to P⋅⋅⋅F pnicogen bonds. There is a correlation between the two bond coupling constants 2hJ(F–F) in the quaternary complexes and the corresponding intermolecular distances, but not in the ternary complexes, a reflection of the distorted geometries of the bridging dimers in ternary complexes.

DOI: 10.1002/chem.201600379

Host–Guest Chemistry: Oxoanion Recognition Based on Combined Charge-Assisted C¢H or Halogen-Bonding Interactions and Anion···Anion Interactions Mediated by Hydrogen Bonds

Several bis-triazolium-based receptors have been synthesized and their anion-recognition capabilities have been studied. The central chiral 1,1′-bi-2-naphthol (BINOL) core features either two aryl or ferrocenyl end-capped side arms with central halogen- or hydrogen-bonding triazolium receptors. NMR spectroscopic data indicate the simultaneous occurrence of several charge-assisted aliphatic and heteroaromatic C−H noncovalent interactions and combinations of C−H hydrogen and halogen bonding. The receptors are able to selectively interact with HP2O73−, H2PO4−, and SO42− anions, and the value of the association constant follows the sequence: HP2O73−>SO42−>H2PO4−. The ferrocenyl end-capped 72+⋅2 BF4− receptor allows recognition and differentiation of H2PO4− and HP2O73− anions by using different channels: H2PO4− is selectively detected through absorption and emission methods and HP2O73− by using electrochemical techniques. Significant structural results are the observation of an anion⋅⋅⋅anion interaction in the solid state (2:2 complex, 62+⋅[H2P2O7]2−), and a short C−I⋅⋅⋅O contact is observed in the structure of the complex [82+][SO4]0.5[BF4].

DOI: 10.1007/s00214-016-1895-8

Competition between intramolecular hydrogen and pnictogen bonds in protonated systems

A theoretical study of the competition between hydrogen (HB) and pnictogen bonds (ZB) in three different families of compounds, (Z)-1,2-disubstituted ethenes (Eth), 1,2-disubstituted benzenes (Phe) and 1,8-disubstituted naphthalenes (Naph), with a charged group, ZH3 + and a neutral one, Z′H2 (Z, Z′ = N, P, As) as interacting moieties, has been carried out. In those structures with a NH3 +motif, intramolecular hydrogen bond structures are minima while pnictogen interactions are transition states. The opposite is true for PH3 + and AsH3 + moieties. An analysis of isodesmic energies (Eiso), interaction energies (E b) and deformation energies (E def) shows that in Eth derivatives, the most stable compound corresponds to P+–N ZB while in the Phe and Naph ones, the N+–N HB interaction presents the largest negative isodesmic energy. Also, Eth and Phe derivatives show negative E iso values for all the compounds under study; however, in some cases of Naphderivatives positive isodesmic energies have been found. The Atoms in Molecules (AIM) analysis of the electron density, natural bond orbital (NBO) second-order orbital energies and electron density shift maps (EDS) have been used to better understand these intramolecular interactions.

DOI: 10.1007/s11224-015-0617-5

A computational study on [(PH₂X)₂]⁺ homodimers involving intermolecular two-center three-electron bonds

A computational study at CCSD(T) theoretical level has been carried out on radical cation [(PH2X)2]·+ homodimers. Four stable minima configurations have been found for seven substituted phosphine derivatives, X = H, CH3, CCH, NC, OH, F and Cl. The most stable minimum presents an intermolecular two-center three-electron P···P bond except for X = CCH. The other three minima correspond to an alternative P···P pnicogen bonded complex, to a P···X contact and the last one to the complex resulting from a proton transfer, PH3X+:PHX·. The complexes obtained have been compared with those of the corresponding neutral ones, (PH2X)2, and the analogous protonated ones, PH3X+:PH2X, recently described in the literature. The spin and charge densities of the complexes have been examined. The electronic characteristics of the complexes have been analyzed with the NBO and AIM methods. The results obtained for the spin density, charge and NBO are coherent for all the complexes.

DOI: 10.1016/j.tet.2016.02.062

Researchgate

Interaction of beryllium derivatives with N-methylated DNA bases: 9-methylguanine and 1-methylcytosine

The effect of beryllium bonds (BeB) on the tautomerism of 9-methyl-guanine and 1-methylcytosine has been theoretically studied at MP2/aug-cc-pVDZ computational level. It is predicted that BeB will be able to strongly modify the tautomeric behaviour of these nucleobases and, by extension, of DNA and RNA bases. Thus, while the oxo-amino is the most stable tautomer for 9-methyl-guanine and 1-methylcytosine when isolated, they present a relative energy above 33 and 17 kJ mol⁻¹, respectively, with respect to the most stable tautomer when they form complexes with BeR₂ systems (amino-hydroxy and imino-oxo, respectively). In both cases, the most stable complexes are associated to N⋯Be interactions with binding energies up to 180 kJ mol⁻¹.

DOI: 10.1039/C6CP00227G

Modulating intramolecular P⋯N pnictogen interactions

A computational study of the intramolecular pnictogen bond in 8-phosphinonaphthalen-1-amine derivatives (1-NX₂, 8-PX₂ with X = H, F, Cl, Br, CH₃, CN and NC), proton sponge analogues, has been carried out to determine their structural and geometric parameters, interaction energies and electronic properties such as the electron density of the intramolecular interaction. Our results show that substitution of H atoms in the PH₂ group by electron withdrawing groups on the Lewis acid moiety strengthens the P⋯N pnictogen bond, evidenced by the increasing electron density values at the bond critical point and by shorter distances. However, substitutions on the Lewis base moiety (NX₂) show weaker P⋯N interactions than when the substitution is done on the Lewis acid counterpart (PX₂). Nevertheless, in all cases, pnictogen bonds are enhanced upon substitution with respect to the parent 1-NH₂, 8-PH₂ system. Second-order orbital interaction energies, electron density maps, electron delocalization functions and charge transfer corroborate the evolution of the P⋯N strength upon substitution.

DOI: 10.1039/C5CP07941A

Researchgate

Halogen bonding. The role of the polarizability of the electron-pair donor

The nature of F–Br⋯X–R interactions (with X = F, Cl, Br, I and R = –H, –F) has been investigated through theoretical calculation of molecular potential electrostatic (MEP), molecular polarizability, atoms in molecules (AIM) analysis and energetic decomposition analysis (EDA). A detailed analysis of the MEPs reveals that considering only the static electrostatic interactions is not sufficient to explain the nature of these interactions. The molecular polarizabilities of X–R molecules suggest that the deformation capacity of the electronic cloud of the lone pairs of the X atom plays an important role in the stability of these complexes. The topological analysis of the L(r) = −¼∇²ρ(r) function and the detailed analysis of the atomic quadrupole moments reveal that the Br⋯X interactions are electrostatic in nature. The electron acceptor Br atom causes a polarization of the electronic cloud (electronic induction) on the valence shell of the X atom. Finally, the electrostatic forces and charge transfer play an important role not only in the stabilization of the complex, but also in the determination of the molecular geometry of equilibrium. The dispersive and polarization forces do not influence the equilibrium molecular geometry.

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DOI: 10.1039/C5CP08046K

ResearchGate

Fullerene and corannulene derivatives acting as insulators of Cl⁻ and BeH₂

The capacity of corannulene and its benzo-derivatives C_xH₁₀ (x = 20–60) as prototypes of non-planar π-aromatic systems which mimic the end of carbon nanotubes to act as insulators between BeH₂ and Cl⁻ chemical entities has been explored by means of M06-2x/cc-pVDZ and M06-2x/6-311+G(d) calculations. For the sake of completeness, the set investigated includes also fullerene, C₆₀. All these aromatic derivatives lead to stable binary complexes either with BeH₂ or halogen (Cl⁻) anions. For BeH₂, however, only the complexes in which the interaction involves the convex face of the aromatic system are stable. No significant changes are observed when the binding energies (BEs) of the BeH₂ complexes are compared to those involving planar aromatic compounds, but the ones involving Cl⁻ with the concave face of the aromatic moiety can be very large, because its curvature favors many contacts of the anion with the carbon atoms of the π-aromatic system. The formation of these binary complexes changes to a large extent the electrostatic potential on the free face of the aromatic system leading to a mutual reinforcement of both interactions, the beryllium bond and the interaction with Cl⁻, when the ternary complexes are formed. As a result, the BEs for the triads are larger than the sum of the BEs of the corresponding binary complexes and the distances between the aromatic subunit and BeH₂ or Cl⁻ become shorter in the triads than in the binary complexes. A MBIE analysis also indicates that the enhanced stability of ternary complexes arises mainly from the reinforcement of the beryllium bonds as well as from the three-body terms. An exploration of all the minima for BeH₂:C₆₀H₁₀:Cl⁻ shows that BeH₂ binds preferentially to the peripheral aromatic rings than those in the more curved region.

DOI: 10.1021/acs.jpca.5b11876

ResearchGate

Cooperativity in Tetrel Bonds

A theoretical study of the cooperativity in linear chains of (H3SiCN)n and (H3SiNC)n complexes connected by tetrel bonds has been carried out by means of MP2 and CCSD(T) computational methods. In all cases, a favorable cooperativity is observed, especially in some of the largest linear chains of (H3SiNC)n, where the effect is so large that the SiH3 group is almost equidistant to the two surrounding CN groups and it becomes planar. In addition, the combination of tetrel bonds with other weak interactions (halogen, chalcogen, pnicogen, triel, beryllium, lithium, and hydrogen bond) has been explored using ternary complexes, (H3SiCN)2:XY and (H3SiNC)2:XY. In all cases, positive cooperativity is obtained, especially in the (H3SiNC)2:ClF and (H3SiNC)2:SHF ternary complexes, where, respectively, halogen and chalcogen shared complexes are formed.

Download from the journal web page, DOI: 10.3390/cryst6020019

H2XP:OH2 Complexes: Hydrogen vs. Pnicogen Bonds

A search of the Cambridge Structural Database (CSD) was carried out for phosphine-water and arsine-water complexes in which water is either the proton donor in hydrogen-bonded complexes, or the electron-pair donor in pnicogen-bonded complexes. The range of experimental P-O distances in the phosphine complexes is consistent with the results of ab initio MP2/aug’-cc-pVTZ calculations carried out on complexes H2XP:OH2, for X = NC, F, Cl, CN, OH, CCH, H, and CH3. Only hydrogen-bonded complexes are found on the H2(CH3)P:HOH and H3P:HOH potential surfaces, while only pnicogen-bonded complexes exist on H2(NC)P:OH2, H2FP:OH2, H2(CN)P:OH2, and H2(OH)P:OH2 surfaces. Both hydrogen-bonded and pnicogen-bonded complexes are found on the H2ClP:OH2 and H2(CCH)P:OH2 surfaces, with the pnicogen-bonded complexes more stable than the corresponding hydrogen-bonded complexes. The more electronegative substituents prefer to form pnicogen-bonded complexes, while the more electropositive substituents form hydrogen-bonded complexes. The H2XP:OH2 complexes are characterized in terms of their structures, binding energies, charge-transfer energies, and spin-spin coupling constants 2hJ(O-P), 1hJ(H-P), and1J(O-H) across hydrogen bonds, and 1pJ(P-O) across pnicogen bonds.

DOI: 10.1016/j.comptc.2015.12.007

The effect of cytosine methylation on its halogen-bonding properties

This study shows the influence of a 5-methyl substituent on the interaction between 1-methylcytosine, 1,5-dimethylcytosine, 1-methyluracil and 1-methylthymine and dichloride (Cl₂) and fluorine chloride (ClF). The methyl derivatives were selected because of the important role played by a methyl group at position 5 of the pyrimidine ring on the biological properties of nucleobases. Besides binding energies obtained at the MP2/aug-cc-pVDZ level, a variety of theoretical methods were used to analyze the structures of the different minima obtained. A total of 116 complexes have been studied.

DOI: 10.1080/00268976.2015.1086835

Properties of cationic pnicogen-bonded complexes F_4-nH_nP⁺:N-base with H–P···N linear and n = 1–4

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the pnicogen-bonded complexes F_4-nH_nP⁺:N-base, for n = 1–4, each with a linear or nearly linear H_ax–P···N alignment. The sp³-hybridised nitrogen bases include NH₃, NClH₂, NFH₂, NCl₂H, NCl₃, NFCl₂, NF₂H, NF₂Cl, and NF_3, and the sp bases are NCNH₂, NCCH₃, NP, NCOH, NCCl, NCH, NCF, NCCN, and N₂. Binding energies increase as the P–N distance decreases, with an exponential curve showing this relationship when complexes with sp³ and sp hybridised bases are treated separately. However, the correlations are not as good as they are for the complexes F_4-nH_nP⁺:N-base for n = 0–3 with F–P···N linear. Different patterns are observed for the change in the binding energies of complexes with a particular base as the number of F atoms in the acid changes. Thus, the particular acid–base pair is a factor in determining the binding energies of these complexes.

Three different charge-transfer interactions stabilise these complexes, namely N_lp→σ*P–H_ax, N_lp→σ*P–F_eq, and N_lp→σ*P–H_eq. Unlike the corresponding complexes with F–P···N linear, N_lp→σ*P–H_ax is not always the dominant charge-transfer interaction, since N_lp→σ*P–F_eq is greater in some complexes. N_lp→σ*P–H_eq makes the smallest contribution to the total charge-transfer energy. The total charge-transfer energies of all complexes increase exponentially as the P–N distance decreases in a manner very similar to that observed for the series of complexes with F–P···N linear.

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin–spin coupling constants ^1pJ(P–N) across the pnicogen bond vary with the P–N distance, but different patterns are observed which depend on the nature of the acid, and for some acids, on the hybridisation of the nitrogen base. ^1pJ(P–N) values for complexes of F₃HP⁺ initially increase as the P–N distance decreases, reach a maximum, and then decrease with decreasing P–N distance as the P···N bond acquires increased covalent character. ^1pJ(P–N) for complexes with H–P···N linear and those with F–P···N linear exhibit similar distance dependencies depending on the number of F atoms in equatorial positions and the hybridisation of the base. Complexation may increase, decrease, or leave the P–H_ax distance unchanged, but¹J(P–H_ax) always decreases relative to the corresponding isolated ion. Decreasing ¹J(P–H_ax) can be related to decreasing intermolecular P–N distance.

DOI: 10.1021/acs.jpca.5b06828

Exploring the (H2C═PH2)+:N-Base Potential Surfaces: Complexes Stabilized by Pnicogen, Hydrogen, and Tetrel Bonds

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to determine the structures, binding energies, and bonding properties of complexes involving the cation (H2C═PH2)+ and a set of sp-hybridized nitrogen bases including NCCH3, NP, NCCl, NCH, NCF, NCCN, and N2. On each (H2C═PH2)+:N-base surface, four types of unique equilibrium structures exist: a complex with a P···N pnicogen bond formed through the π system of (H2C═PH2)+ (ZB-π); a complex with a P···N pnicogen bond formed through the σ system of (H2C═PH2)+ (ZB-σ); a hydrogen-bonded complex with a P—H···N hydrogen bond (HB); and a tetrel-bonded complex with a C···N bond (TB). Binding energies of complexes stabilized by the same type of intermolecular interaction decrease in the order NCCH3 > NP > NCCl > NCH > NCF > NCCN > N2. For a given base, binding energies decrease in the order ZB-π > HB > ZB-σ > TB, except for a reversal of HB and ZB-σ with the weakest base N2. Binding energies of ZB-π, HB, and ZB-σ complexes increase exponentially as the corresponding P—N distance decreases, but the correlation is not as good between the binding energies of TB complexes and the intermolecular C—N distance. Charge-transfer energies stabilize all complexes and also exhibit an exponential dependence on the corresponding intermolecular distances. EOM-CCSD spin–spin coupling constants 1pJ(P—N) for ZB-π and ZB-σ complexes, and 2hJ(P—N) for HB complexes increase quadratically as the corresponding P—N distance decreases. Values of 1tJ(C—N) for TB are small and show little dependence on the C—N distance. 1J(P—H) values for the hydrogen-bonded P—H bond in HB complexes correlate with the corresponding P—H distance, whereas values of 1J(P—H) for the non-hydrogen-bonded P—H correlate with the P—N distance.

DOI: 10.1039/C5CP05832E

Can HN=NH, FN=NH, or HN=CHOH bridge the σ-hole and the lone pair at P in binary complexes with H2XP, for X = F, Cl, NC, OH, CN, CCH, CH3, and H?

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the properties of complexes formed between H₂XP, for X = F, Cl, NC, OH, CN, CCH, CH₃, and H, and the possible bridging molecules HN═NH, FN═NH, and HN═CHOH. H₂XP:HNNH and H₂XP:FNNH complexes are stabilized by P⋯N pnicogen bonds, except for H₂(CH₃)P:FNNH and H₃P:FNNH which are stabilized by N–H⋯P hydrogen bonds. H₂XP:HNCHOH complexes are stabilized by P⋯N pnicogen bonds and nonlinear O–H⋯P hydrogen bonds. For a fixed H₂XP molecule, binding energies decrease in the order HNCHOH > HNNH > FNNH, except for the binding energies of H₂(CH₃)P and H₃P with HNNH and FNNH. Binding energies of complexes with HNCHOH and HNNH increase as the P–N₁ distance decreases, but binding energies of complexes with FNNH show little dependence on this distance. The large binding energies of H₂XP:HNCHOH complexes arise from a cooperative effect involving electron-pair acceptance by P to form a pnicogen bond, and electron-pair donation by P to form a hydrogen bond. The dominant charge-transfer interaction in these complexes involves electron-pair donation by N across the pnicogen bond, except for complexes in which X is one of the more electropositive substituents, CCH, CH₃, and H. For these, lone-pair donation by P across the hydrogen bond dominates. AIM and NBO data for these complexes are consistent with their bonding characteristics, showing molecular graphs with bond critical points and charge-transfer interactions associated with hydrogen and pnicogen bonds. EOM-CCSD spin–spin coupling constants ^1pJ(P–N) across the pnicogen bond for each series of complexes correlate with the P–N distance. In contrast, ^2hJ(O–P) values for complexes H₂XP:HNCHOH do not correlate with the O–P distance, a consequence of the nonlinearity of these hydrogen bonds.

DOI: 10.1016/j.cplett.2015.10.050

Exploring the PX3:NCH and PX3:NH3 potential surfaces, with X = F, Cl, and Br

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to explore the PX3:NCH and PX3:NH3 potential surfaces, with X = F, Cl and Br. Four unique minima exist on the PX3:NCH surfaces, and three on the PX3:NH3 surfaces. Complexes stabilized by pnicogen bonds, hydrogen bonds, halogen bonds, and electrostatic interactions have been found at these minima. The global minimum on each surface is a complex with a P···N pnicogen bond. Binding energies at corresponding minima are ordered with respect to the PX3 molecules as PF3 < PCl3 < PBr3. Charge-transfer stabilizes all complexes with intermolecular bonds.

DOI: 10.1002/chem.201500981

Creating σ-Holes through the Formation of Beryllium Bonds

Through the use of ab initio theoretical models based on MP2/aug-cc-pVDZ-optimized geometries and CCSD(T)/aug-cc-pVTZ and CCSD(T)/aug-c-pVDZ total energies, it has been shown that the significant electron density rearrangements that follow the formation of a beryllium bond may lead to the appearance of a σ-hole in systems that previously do not exhibit this feature, such as CH₃OF, NO₂F, NO₃F, and other fluorine-containing systems. The creation of the σ-hole is another manifestation of the bond activation–reinforcement (BAR) rule. The appearance of a σ-hole on the F atoms of CH₃OF is due to the enhancement of the electronegativity of the O atom that participates in the beryllium bond. This atom recovers part of the charge transferred to Be by polarizing the valence density of the F into the bonding region. An analysis of the electron density shows that indeed this bond becomes reinforced, but the F atom becomes more electron deficient with the appearance of the σ-hole. Importantly, similar effects are also observed even when the atom participating in the beryllium bond is not directly attached to the F atom, as in NO₂F, NO₃F, or NCF. Hence, whereas the isolated CH₃OF, NO₂F, and NO₃F are unable to yield F⋅⋅⋅Base halogen bonds, their complexes with BeX₂ derivatives are able to yield such bonds. Significant cooperative effects between the new halogen bond and the beryllium bond reinforce the strength of both noncovalent interactions.

DOI: 10.1039/C5NJ00600G

Halogen, chalcogen and pnictogen interactions in (XNO₂)₂ homodimers (X = F, Cl, Br, I)

A theoretical study of the XNO₂ homodimers (X = F, Cl, Br and I) has been carried out by means of the Møller–Plesset (MP2) methodology. Twenty-two different minimum structures have been found, involving pnictogen, chalcogen and halogen bonds. MP2 interaction energies range between −0.4 to −17.5 kJ mol⁻¹. Atoms in molecules (AIM) and natural bond orbital (NBO) approaches have been used to analyse the nature of the interaction within both monomers, obtaining good correlations between Laplacian values and bond distances. NBO E(2) orbital interaction energies are found to be up to 39.0 kJ mol⁻¹. Charge transfer between monomers is in agreement with those in AIM and NBO findings, showing the highest charge transferred in those asymmetric dimers which involve pure halogen bonds. Symmetry adapted perturbation theory (SAPT-DFT) results show that the interactions are driven by the dispersion term, followed by the electrostatic one. The induction term presents the lowest contribution with the exception of complexes 1 and 5 of the iodine derivative in which E⁽²⁾_i shows the maximum contribution to the total forces.

DOI: 10.1002/cphc.201500273

Simultaneous Aromatic–Beryllium Bonds and Aromatic–Anion Interactions: Naphthalene and Pyrene as Models of Fullerenes, Carbon Single-Walled Nanotubes, and Graphene

The possibility of forming stable BeR₂:ArH:Y⁻ (R=H, F, Cl; ArH=naphthalene, pyrene; Y=Cl, Br) ternary complexes in which the beryllium compounds and anions are located on the opposite sides of an extended aromatic system is explored by means of MP2/aug-cc-pVDZ ab initio calculations. Comparison of the electron-density distribution of these ternary complexes with the corresponding BeR₂:ArH and ArH:Y⁻binary complexes reveals the existence of significant cooperativity between the two noncovalent interactions in the triads. The energetic effects of this cooperativity are quantified by evaluation of the three-body interaction energy Δ³E in the framework of the many-body interaction-energy (MBIE) approach. Although an essential component of the interaction energies is electrostatic and is well reflected in the changes in the molecular electrostatic potential of the aromatic system on complexation, strong polarization effects, in particular for the BeR₂:ArH interactions, also play a significant role. The charge transfers associated with these polarization effects are responsible for significant distortion of both the BeR₂ and the aromatic moieties. The former are systematically bent in all the complexes, and the latter are curved to a degree that depends on the nature of the R substituents of the BeR₂ subunit.

DOI: 10.1016/j.jorganchem.2015.07.013 

Theoretical study of the geometrical, energetic and NMR properties of atranes

Theoretical calculations of 87 atranes, 113 considering the different endo/exoconformations, with combinations of B, C, N, Al, Si, P and Ge atoms at the bridgehead positions have been performed at the B3LYP/6-311++G(d,p) level. Using the optimized minimum energy geometries we have calculated their chemical shifts (GIAO approximation) and their indirect spin–spin coupling constants J as well as the reduced coupling constants K. Besides, a topological analysis was carried out with the Atoms in Molecules Theory (AIM). When available, experimental data will be reported and compared with calculations.

DOI: 10.1016/j.tet.2015.06.023

The influence of intermolecular halogen bonds on the tautomerism of nucleobases. I. Guanine 

In this paper, we report a computational study of the influence of halogen bonds (XB) on the tautomerism of 9-methylguanine, a model of guanosine and GTP. Molecular Electrostatic Potentials (MEP), interaction energies, Atoms in Molecules (AIM) and Natural Bond Orbitals (NBO) were used to characterize the complexes between three tautomers of 9-methylguanine (one oxo and two hydroxy) and five dihalogens molecules (Br₂, BrCl, BrF, Cl₂ and ClF). The results concerning the halogen bonds were compared with the effect of hydrogen bonds using hydrogen fluoride as a hydrogen bond donor. The binding energies of the complexes have been analyzed using a Free-Wilson model to obtain the individual interactions. An important conclusion of this study is that halogen bonds are able to strongly modify the order of stability of the three most stable tautomers that, in turn, could induce mutations according to the ‘rare tautomer hypothesis’.

DOI: 10.1002/chem.201500231

Dual Role of the 1,2,3-Triazolium Ring as a Hydrogen-Bond Donor and Anion–π Receptor in Anion-Recognition Processes

Several bis(triazolium)-based receptors have been synthesized as chemosensors for anion recognition. The central naphthalene core features two aryltriazolium side-arms. NMR experiments revealed differences between the binding modes of the two triazolium rings: one triazolium ring acts as a hydrogen-bond donor, the other as an anion–π receptor. Receptors 9²⁺⋅2BF₄⁻ (C₆H₅), 11²⁺⋅2BF₄⁻ (4-NO₂C₆H₄), and 13²⁺⋅2BF⁴⁻ (ferrocenyl) bind HP₂O₇³⁻ anions in a mixed-binding mode that features a combination of hydrogen-bonding and anion–π interactions and results in strong binding. On the other hand, receptor 10²⁺⋅2 BF₄⁻ (4-CH₃OC₆H₄) only displays combined Csp₂H/anion–π interactions between the two arms of the receptors and the bound anion rather than triazolium (CH)⁺⋅⋅⋅anion hydrogen bonding. All receptors undergo a downfield shift of the triazolium protons, as well as the inner naphthalene protons, in the presence of H₂PO₄⁻ anions. That suggests that only hydrogen-bonding interactions exist between the binding site and the bound anion, and involve a combination of cationic (triazolium) and neutral (naphthalene) CH donor interactions. Theoretical calculations relate the electronic structure of the substituent on the aromatic group with the interaction energies and provide a minimum-energy conformation for all the complexes that explains their measured properties.

DOI: 10.1021/acs.jpca.5b03035

Properties of Cationic Pnicogen-Bonded Complexes F_4–nH_nP⁺:N-Base with F–P···N Linear and n = 0–3

Ab initio MP2/aug′-cc-pVTZ calculations were performed to investigate the pnicogen-bonded complexes F4–nHnP+:N-base, for n = 0–3, each with a linear or nearly linear F–P···N alignment. The nitrogen bases include the sp3 bases NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3 and the sp bases NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2. The binding energies vary between −20 and −180 kJ·mol–1, while the P–N distances vary from 1.89 to 3.01 Å. In each series of complexes, binding energies decrease exponentially as the P–N distance increases, provided that complexes with sp3 and sp hybridized bases are treated separately. Different patterns are observed for the change in the binding energies of complexes with a particular base as the number of F atoms in the acid changes. Thus, the particular acid–base pair is a factor in determining the binding energies of these complexes. Three different charge-transfer interactions stabilize these complexes. These arise from the nitrogen lone pair to the σ*P–Fax, σ*P–Feq, and σ*P–Heq orbitals. The dominant single charge-transfer energy in all complexes is Nlp → σ*P–Fax. However, since there are three Nlp → σ*P–Feq charge-transfer interactions in complexes with F4P+ and two in complexes with F3HP+, the sum of the Nlp → σ*P–Feq charge-transfer energies is greater than the Nlp → σ*P–Fax charge-transfer energies in the former complexes, and similar to the Nlp → σ*P–Fax energies in the latter. The total charge-transfer energies of all complexes decrease exponentially as the P–N distance increases. Coupling constants 1pJ(P–N) across the pnicogen bond vary with the P–N distance, but different patterns are observed for complexes with F4P+ and complexes of the sp3 bases with F3HP+. These initially increase as the P–N distance decreases, reach a maximum, and then decrease with decreasing P–N distance as the P···N bond acquires increased covalent character. For the remaining complexes, 1pJ(P–N) increases with decreasing P–N distance. Complexation increases the P–Fax distance and 1J(P–Fax) relative to the corresponding isolated ion. 1J(P–Fax) correlates quadratically with the P–N distance.

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Interplay between Beryllium Bonds and Anion-π Interactions in BeR₂:C₆X₆:Y− Complexes (R = H, F and Cl, X = H and F, and Y = Cl and Br)

A theoretical study of the beryllium bonds in BeR2:C6X6 (R = H, F, Cl and X = H and F) has been carried out by means of MP2/aug′-cc-pVDZ computational methods. In addition, the ternary complexes BeR2:C6X6:Y− (Y = Cl and Br) have been analyzed. Geometric, energetic and electronic aspects of the complexes have been taken into account. All the parameters analyzed provide a clear indication of favorable cooperativity in both interactions observed, beryllium bond and aromatic ring:anion interaction.

DOI: 10.1007/978-3-319-14163-3_8

The Pnicogen Bond in Review: Structures, Binding Energies, Bonding Properties, and Spin-Spin Coupling Constants of Complexes Stabilized by Pnicogen Bonds

Extensive ab initio MP2/aug’-cc-pVTZ studies have been carried out in our laboratories to determine the structures, binding energies, bonding properties, and EOM-CCSD spin-spin coupling constants of various series of complexes stabilized by pnicogen bonds.  These systematic studies provide insight into the nature of the pnicogen bond, and how changes in this bond are reflected in the properties of these complexes.

DOI: 10.1002/chir.22438

Adding Only One Priority Rule Allows Extending CIP Rules to Supramolecular Systems

There are frequent situations both in supramolecular chemistry and in crystallography that result in stereogenic centers, whose absolute configuration needs to be specified. With this aim we propose the inclusion of one simple additional rule to the Cahn-Ingold-Prelog (CIP) system of priority rules stating that noncovalent interactions have a fictitious number between 0 and 1. 

DOI: 10.1021/jp511118s

Double Hole–Lump Interaction between Halogen Atoms

In this paper a theoretical study has been carried out to investigate the nature of the unusual halogen–halogen contacts in the complexes R–X···X–R (with R = −H, −Cl, −F and X = Cl, Br, I). AIM, NBO, and MEP analyses have been used to characterize X···X interactions. Formation of the unusual X···X interactions leads to a significant increase of electron charge density in the bonding region between the two halogen atoms. The geometry and stability of these complexes is mainly due to electrostatic interactions lump(X1) → hole(X2) and lump(X2) → hole(X1) [or equivalently [VS,min(X1) → VS,max(X2) and VS,min(X2) → VS,max(X1)] and the charge transfers LP(X1) → σ*(R–X2) and LP(X2) → σ*(R–X1). In other words, these findings suggest that the electrostatic interactions and the charge transfer play a substantial role in determining the optimal geometry of these complexes, as in conventional halogen bonds, even though the dispersion term is the most important attractive term for all the complexes studied here, save one.

DOI: 10.1007/s11224-015-0581-0

The influence of halogen bonds on tautomerism: the case of 3-mercapto-1,2-azoles (pyrazoles, isoxazoles, isothiazoles)

DFT calculations at the B3LYP/6-311++G(d,p) computational level have been carried out on three tautomeric pairs of 3-mercapto-1,2-azoles (pyrazoles, isoxazoles, and isothiazoles) to study the effect of halogen bonds (XBs) on the position of the equilibrium. As halogen bond donors, we have selected Br2, Cl2, BrCl, ClF and BrF and compare them with HF as a hydrogen bond donor. Several linear relationships were found between binding energies of different halogen bond donors. The main conclusion of this study is that the XB inverts the tautomeric equilibrium while an HB does not.

DOI: 10.1021/acs.jpca.5b00944

P···N Pnicogen Bonds in Cationic Complexes of F₄P⁺ and F₃HP⁺ with Nitrogen Bases

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out on cationic pnicogen-bonded complexes F4P+:N-base and F3HP+:N-base, with linear Fax–P···N and Hax-P···N, respectively. The bases include the sp3-hybridized nitrogen bases NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3, and the sp bases NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2. The binding energies of these complexes span a wide range, from −15 to −180 kJ mol–1, as do the P–N distances, which vary from 1.89 to 3.11 Å. There is a gap in the P–N distances between 2.25 and 2.53 Å in which no complexes are found. Thus, the equilibrium complexes may be classified as inner or outer complexes based on the value of the P–N distance. Inner complexes have P···N bonds with varying degrees of covalent character, whereas outer complexes are stabilized by intermolecular P···N bonds with little or no covalency. Charge-transfer stabilizes these pnicogen-bonded complexes. For complexes F4P+:N-base, the dominant charge-transfer interaction is from the lone pair on N to the σ*P–Faxorbital. In addition, there are three other charge-transfer interactions from the lone pair on N to the σ*P–Feq orbitals, which taken together, are more stabilizing than the interaction involving σ*P–Fax. In contrast, the dominant charge-transfer interaction for complexes F3HP+:N-base is from the lone pair on N to the σ*P–Feq orbitals. Computed EOM-CCSD Fermi-contact terms are excellent approximations to the total spin–spin coupling constants 1pJ(P–N) and 1J(P–Hax), but are poor approximations to 1J(P–Fax). 1pJ(P–N) values increase with decreasing P–N distance, approach a maximum, and then decrease and change sign as the P–N distance further decreases and the pnicogen bond acquires increased covalency. 1J(P–Fax) values for F4P+:N-base complexes increase with decreasing distance. Although the P–Hax distance changes very little in complexes F3HP+:N-base, patterns exist which suggest that changes in 1J(P–Hax) reflect the hybridization of the nitrogen base and whether the complex is an inner or outer complex.

DOI: 10.1007/s00214-015-1630-x

Ab initio study in the hydration process of metaphosphoric acid: the importance of the pnictogen interactions

A theoretical study of the hydration of metaphosphoric acid to yield phosphoric acid has been carried out by means of MP2/6-31+G(d,p) and MP2/aug-cc-pVTZ computational levels. Up to three explicit water molecules have been considered as well as the PCM solvation model to account for the effect of the bulk water. The reaction profile has been analyzed using the conceptual DFT methodology. The reactant structure is very dependent on the number of water molecules. The inclusion of more than one water molecule produces important cooperative effects and a shortening of the O···P pnictogen interaction besides the reaction barrier drops about 50 kJ mol−1. Reaction force at ξ 1 indicates the decreasing in the angular stress in the reaction site before reaching the TS as more explicit water molecules are taken into account. The analysis of the reaction electronic flux shows that for the three mechanisms studied, the principal reactive changes occur in the TS zone, while reactants and products remain in a zero-flux regime.

DOI: 10.1021/jp511828h

Chalcogen Bonds in Complexes of SOXY (X, Y = F, Cl) with Nitrogen Bases

SOF2, SOFCl, and SOCl2 were each paired with a series of N bases. The potential energy surface of the binary complexes were characterized by MP2 calculations with double and triple-ξ basis sets, extrapolated to complete sets. The most stable configurations contained a S···N chalcogen bond with interaction energies as high as 6.8 kcal/mol. These structures are stabilized by a Nlp → σ*(S–Z) electron transfer (Z = O, F, Cl), complemented by Coulombic attraction of N to the σ-hole opposite the Z atom. N···S–F and N···S–Cl chalcogen bonds are stronger than N···S═O interactions. Formation of each chalcogen bond elongates all of the internal covalent bonds within SOXY, especially the S–Cl bond. Halogen-bonded (N···Cl–S) complexes were also observed, but these are more weakly bound, by less than 3 kcal/mol.

DOI: 10.1039/C4CP04840G

Pnicogen and hydrogen bonds: complexes between PH₃X^₊ and PH₂X systems

The charge-assisted complexes between PH₃X⁺ and PH₂X have been analyzed. MP2/aug′-cc-pVTZ calculations were performed and the results were supported by the Quantum Theory of Atoms in Molecules approach and the Natural Bond Orbitals method. It was found that three different configurations could be formed, i.e. those linked through a PP or a PX pnicogen bond and those linked through a P–HP hydrogen bond. The PP configurations are the most stable ones corresponding to the strongest interactions; for all complexes the PP configuration exists, while the PX and P–HP ones are present only for some of them. Different relations between the parameters were found, especially for the PP interactions where there are correlations between the PP distance and the electron density at the PP bond critical point (ρ_PP) as well as between ρ_PP and the charge transfer energy.

DOI: 10.1016/j.comptc.2014.07.009

Theoretical study of cyanophosphines: Pnicogen vs. dipole–dipole interactions

Cyanophosphine derivative dimers, [HXP(CN)]₂ with X = H, F and Cl, have been characterized by means of CCSD(T)/aug′-cc-pVTZ//MP2/aug′-cc-pVTZ computational level calculations. Different interactions have been found upon complexation, such as hydrogen bonds, pnicogen bonds and dipole···dipole interactions. The intermolecular distances range between 2.84 and 3.53 Å and the binding energies between −34.7 and −3.6 kJ mol⁻¹. Compounds with dipole···dipole interactions present shorter contact distances and larger (more negative) binding energies than those with pure P···P pnicogen bonds. Electron density shift maps show larger variations in compounds with dipole···dipole interactions than in those with pure pnicogen ones, in line with the energetic results. However, NBO analysis suggests that the complexes with P···P pnicogen bonds, in special those with XP···PX (X = F, Cl) show E(2) orbital interaction energies much larger than the dipole···dipole ones.

DOI: 10.1021/jp5117504

Substituent Effects on the Properties of Pnicogen-Bonded Complexes H2XP:PYH2, for X, Y = F, Cl, OH, NC, CCH, CH3, CN, and H

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out on the pnicogen-bonded homodimers (PH2X)2 and the binary complexes H2XP:PYH2, for X, Y = F, Cl, OH, NC, CCH, CH3, CN, and H. The binding energies of these complexes are influenced by the nature of the X,Y pair, the intermolecular distance, the relative orientation of the interacting molecules, and the charge-transfer energies from the lone pair of one P to the σ-hole of the other. Binary complexes with X,Y = F, Cl, OH, and NC, as well as the homodimers, have a trans arrangement of the P–A and P–A′ bonds with respect to the P···P bond, with A and A′, the atoms of X and Y, respectively, bonded to the P atoms. The trendlines for the homodimers in plots of the binding energy versus the P–P distance, and the binding energy versus the total charge-transfer energy, exhibit better correlations than the trendlines for the binary complexes. The trendlines for the homodimers mark the boundary of the region in which points for the binary complexes appear. Pnicogen-bond radii for P in PH2X molecules have been determined from the P–P distances in the homodimers. The sum of these radii provides an excellent approximation to the P–P distance in the corresponding binary complex. EOM-CCSD spin–spin coupling constants 1pJ(P–P) have also been computed for all complexes. Coupling constants for the dimers and binary complexes exhibit a similar linear increase as the P–P distance decreases.

DOI: 10.1021/jp510198g

The Paradox of Hydrogen-Bonded Anion–Anion Aggregates in Oxoanions: A Fundamental Electrostatic Problem Explained in Terms of Electrophilic···Nucleophilic Interactions

A theoretical study of anionic complexes formed by two partly deprotonated oxoacids joined by hydrogen bonds has been carried out at the MP2 computational level. In spite of the ionic repulsion, local energy minima are found both in the gas phase and in aqueous solution. Electrostatic potential and electron density topologies, and the comparison with neutral complexes formed by oxoacids, reveal that the ionization has no significant effect on the properties of the hydrogen bonds. The stability of the complexes in the gas phase is explained by attractive forces localized in a volume situated in the hydrogen bond and defined as the electrostatic attraction region (EAR) and determined by the topological analyses of the electron density and the electrostatic potential, and by the electric field lines. In solution, the strong anionic repulsion is mostly screened by the effect of the surrounding polar solvent, which only leads to a weak destabilizing interaction in the hydrogen bond region and finally favors the overall stability of the complexes. The anion–anion complexes have been compared with the corresponding neutral ones (as salts or protonated forms), showing that EAR remains unchanged along the series.

DOI: 10.1039/c4cp04574b

Using Beryllium Bonds to Change Halogen Bonds From Traditional to Chlorine-shared to Ion-pair

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the structures, binding energies, and bonding characteristics of binary complexes HFBe:FCl, R₂Be:FCl, and FCl:N-base, and of ternary complexes HFBe:FCl:N-base and R₂Be:FCl:N-base for R = H, F, Cl; N-base = NH₃, NHCH₂, NCH. Dramatic synergistic cooperative effects have been found between the Be···F beryllium bonds and the Cl···N halogen bonds in ternary complexes. The Cl···N traditional halogen bonds and the Be···F beryllium bonds in binary complexes become significantly stronger in ternary complexes, while the F–Cl bond weakens. Charge-transfer from F to the empty p(σ) orbital of Be leads to a bending of the XYBe molecule and a change in the hybridization of Be, which in the limit becomes sp². As a function of the intrinsic basicity of the nitrogen base and the intrinsic acidity of the Be derivative, the halogen-bond type evolves from traditional to chlorine-shared to ion-pair bonds. The mechanism by which an ion-pair complex is formed is similar to that involved in the dissociative proton attachment process. EOM-CCSD spin–spin coupling constants ^1XJ(Cl–N) across the halogen bond in these complexes also provide evidence of the same evolution of the halogen-bond type.

DOI: 10.5562/cca2458

Some Interesting Features of Non-Covalent Interactions

Interactions between closed-shell systems exhibit some common features, four of which are particularly strong for beryllium bonds: geometrical distortion, cooperativity, changes in intrinsic reactivity and changes in the magnetic properties of the interacting subunits, which reflect the perturbations of their electron densities through polarization effects. Structural changes lead to interaction energies that can only be adequately accounted for when the effects of the distortion on the intrinsic reactivity of the system, and not only its deformation energy, are taken into consideration. Self-assembling of ditopic systems may lead to n-mers stabilized by strong cooperative effects. Chemical shifts and coupling constants also reflect the perturbations of the electron density and accordingly cooperative effects. These four features are common to any interaction involving two closed-shell systems, one acting as Lewis acid and the other as Lewis base, and the only difference between the nature of the interactions is quantitative.

DOI: 10.1007/s11224-014-0484-5

A theoretical study of the thermodynamic and hydrogen-bond basicity of TEMPO radical and related nitroxides

The use of B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) calculations on TEMPO and four related nitroxide radicals having another oxygen functionality (ketone, hydroxyl and ether) has allowed to determine the relative basicities (thermodynamic and hydrogen-bonded) of the oxygen lone pair relative to the nitroxide radical. The differences are small, especially the B3LYP ones, but in all cases they favor the radical. This is consistent with experimental results in the case of the hydroxyl group but not in the case of the keto group.

DOI: 10.1021/jp509353a

Pnicogen-Bonded Complexes H_nF_5–nP:N-Base, for n = 0–5

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out on the pnicogen-bonded complexes HnF5–nP:N-base, for n = 0–5 and nitrogen bases NC–, NCLi, NP, NCH, and NCF. The structures of these complexes have either C4v or C2v symmetry with one exception. P–N distances and interaction energies vary dramatically in these complexes, while Fax–P–Feqangles in complexes with PF5 vary from 91° at short P–N distances to 100° at long distances. The value of this angle approaches the Fax–P–Feq angle of 102° computed for the Berry pseudorotation transition structure which interconverts axial and equatorial F atoms of PF5. The computed distances and Fax–P–Feq angles in complexes F5P:N-base are consistent with experimental CSD data. For a fixed acid, interaction energies decrease in the order NC– > NCLi > NP > NCH > NCF. In contrast, for a fixed base, there is no single pattern for the variations in distances and interaction energies as a function of the acid. This suggests that there are multiple factors that influence these properties. The dominant factor appears to be the number of F atoms in equatorial positions, and then a linear Fax–P···N rather than Hax–P···N alignment. The acids may be grouped into pairs (PF5, PHF4) with four equatorial F atoms, then (PH4F, PH2F3) with Fax–P···N linear, and then (PH3F2 and PH5) with Hax–P···N linear. The electron-donating ability of the base is also a factor in determining the structures and interaction energies of these complexes. Charge transfer from the N lone pair to the σ* P–Aax orbital stabilizes HnF5–nP:N-base complexes, with Aax either Fax or Hax. The total charge-transfer energies correlate with the interaction energies of these complexes. Spin–spin coupling constants 1pJ(P–N) for (PF5, PHF4) complexes with nitrogen bases are negative with the strongest bases NC– and NCLi but positive for the remaining bases. Complexes of (PH4F, PH2F3) with these same two strong bases and H4FP:NP have positive 1pJ(P–N) values but negative values for the remaining bases. (PH5, PH3F2) have negative values of 1pJ(P–N) only for complexes with NC–. Values of 1J(P–Fax) and 1J(P–Hax) correlate with the P–Fax and P–Haxdistances, respectively.

DOI: 10.1007/s00214-014-1586-2

Noncovalent interactions in dimers and trimers of SO3 and CO

The SO3:CO heterodimer has been found by ab initio calculations to form a complex in which the C lone pair of CO interacts with the π*(SO) antibond via the π-hole lying directly above the S atom of SO3. The binding energy of this complex is 4.3 kcal/mol, with Coulombic attraction as its main component. There is also a secondary minimum, with half that strength, wherein the CO molecule is rotated so that it is its O atom that interacts with SO3. The most stable SO3:(CO)2 heterotrimer has the two CO molecules approaching the S atom from above and below the SO3 plane with the C atoms of the CO interacting with the S of the SO3. A strong chalcogen bond between SO3 molecules is the dominant feature of the (SO3)2:CO trimer, supplemented by a S···C chalcogen bond in the SO3:CO dimer.

DOI: 10.1021/jp506663x

Traditional and Ion-Pair Halogen-Bonded Complexes Between Chlorine and Bromine Derivatives and a Nitrogen-Heterocyclic Carbene

A theoretical study of the halogen-bonded complexes (A–X···C) formed between halogenated derivatives (A–X; A = F, Cl, Br, CN, CCH, CF3, CH3, H; and X = Cl, Br) and a nitrogen heterocyclic carbene, 1,3-dimethylimidazole-2-ylidene (MeIC) has been performed using MP2/aug′-cc-pVDZ level of theory. Two types of A–X:MeIC complexes, called here type-I and -II, were found and characterized. The first group is described by long C–X distances and small binding energies (8–54 kJ·mol–1). In general, these complexes show the traditional behavior of systems containing halogen-bonding interactions. The second type is characterized by short C–X distances and large binding energies (148–200 kJ·mol–1), and on the basis of the topological analysis of the electron density, they correspond to ion-pair halogen-bonded complexes. These complexes can be seen as the interaction between two charged fragments: A– and +[X–CIMe] with a high electrostatic contribution in the binding energy. The charge transfer between lone pair A(LP) to the σ* orbital of C–X bond is also identified as a significant stabilizing interaction in type-II complexes.

DOI: 10.1039/C4CP02380C

Strongly bound noncovalent (SO₃)_n:H₂CO complexes (n = 1, 2)

The potential energy surfaces (PES) for the SO₃:H₂CO and (SO₃)₂:H₂CO complexes were thoroughly examined at the MP2/aug-cc-pVDZ computational level. Heterodimers and trimers are held together primarily by SO chalcogen bonds, supplemented by weaker CHO and/or OC bonds. The nature of the interactions is probed by a variety of means, including electrostatic potentials, AIM, NBO, energy decomposition, and electron density redistribution maps. The most stable dimer is strongly bound, with an interaction energy exceeding 10 kcal mol⁻¹. Trimers adopt the geometry of the most stable dimer, with an added SO₃ molecule situated so as to interact with both of the original molecules. The trimers are strongly bound, with total interaction energies of more than 20 kcal mol⁻¹. Most such trimers show positive cooperativity, with shorter SO distances, and three-body interaction energies of nearly 3 kcal mol⁻¹.

DOI: 10.1021/jo501061z

Open Bis(triazolium) Structural Motifs as a Benchmark To Study Combined Hydrogen- and Halogen-Bonding Interactions in Oxoanion Recognition Processes

We have designed a series of triazolium-pyrene-based dyads to probe their potential as fluorescent chemosensors for anion recognition through combinations of hydrogen and halogen bonding. Cooperation between the two distinct noncovalent interactions leads to an unusual effect on receptor affinity, as a result of fundamental differences in the interactions of halogen and hydrogen bond donor groups with anions. Absorption, emission spectrophotometries and proton and phosphorus NMR spectroscopies indicate that the two interactions act in concert to achieve the selective binding of the hydrogen pyrophosphate anion, a conclusion supported by computational studies. Hence, as clearly demonstrated with respective halogen- and hydrogen-bonding triazolium receptors, the integration of a halogen atom into the anion receptor at the expense of one hydrogen-bonding receptor greatly influences the anion recognition affinity of the receptor. The association constant values of the halogen-bonding complexes are larger than the hydrogen-bonding counterpart. Thus, halogen bonding has been exploited for the selective fluorescent sensing of hydrogen pyrophosphate anion. Halogen bonding has been demonstrated to increase the strength of hydrogen pyrophosphate binding, as compared to the hydrogen-bonded analogue. Grimme’s PBE-D functional, which adequately reproduces the pyrene stacking energies, has been successfully applied to model the affinity for anions, especially hydrogen pyrophosphate, of the new receptors.

DOI: 10.1039/C4CP01072H

Intramolecular pnicogen interactions in phosphorus and arsenic analogues of proton sponges

A computational study of the intramolecular pnicogen bond in 1,8-bis-substituted naphthalene derivatives (ZXH and ZX₂ with Z = P, As and X = H, F, Cl, and Br), structurally related to proton sponges, has been carried out. The aim of this paper is the study of their structural parameters, interaction energies and electronic properties such as electron density on the intramolecular interaction. The calculated geometrical parameters associated to the PP interaction are in reasonably good agreement with the crystal structures found in a CSD search, in particular those of the halogen derivatives. Isodesmic reactions where the 1,8-bis-substituted derivatives are compared to monosubstituted derivatives have been calculated, indicating that the 1,8 derivatives are more stable than the monosubstituted ones for those cases with X–ZZ–X and F–ZZ–H alignments. Electron densities and Laplacians at the BCP on the pnicogen interactions suggest that they can be classified as pure closed shell interactions with a partial covalent character. Electron density shift maps are consistent with the results for intermolecular pnicogen interactions. Relationships between interatomic distance and electron density at the bond critical points and between interatomic distance and the orbital charge transfer stabilization energies have been found.

DOI: 10.1063/1.4884962

An exploration of the ozone dimer potential energy surface

The (O3)2 dimer potential energy surface is thoroughly explored at the ab initio CCSD(T) computational level. Five minima are characterized with binding energies between 0.35 and 2.24 kcal/mol. The most stable may be characterized as slipped parallel, with the two O3monomers situated in parallel planes. Partitioning of the interaction energy points to dispersion and exchange as the prime contributors to the stability, with varying contributions fromelectrostatic energy, which is repulsive in one case. Atoms in Molecules analysis of thewavefunction presents specific O⋯O bonding interactions, whose number is related to the overall stability of each dimer. All internal vibrational frequencies are shifted to the red by dimerization, particularly the antisymmetric stretching mode whose shift is as high as 111 cm⁻¹. In addition to the five minima, 11 higher-order stationary points are identified.

DOI: 10.1021/jp503436f

Characterizing Traditional and Chlorine-Shared Halogen Bonds in Complexes of Phosphine Derivatives with ClF and Cl2

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out on the halogen-bonded complexes H2XP:ClF and H2XP:Cl2, with X = F, Cl, OH, NC, CN, CCH, CH3, and H. H2XP:ClF complexes are stabilized by chlorine-shared halogen bonds with short P–Cl and significantly elongated Cl–F distances. H2XP:Cl2 complexes with X = OH and CH3 form only chlorine-shared halogen bonds, while those with X = H, NC, and CN form only traditional halogen bonds. On the H2FP:Cl2, H2(CCH)P:Cl2, and H2ClP:Cl2 potential surfaces small barriers separate two equilibrium structures, one with a traditional halogen bond and the other with a chlorine-shared bond. The binding energies of H2XP:ClF and H2XP:Cl2 complexes are influenced by the electron-donating ability of H2XP and the electron accepting ability of ClF and ClCl, the nature of the halogen bond, other secondary interactions, and charge-transfer interactions. Changes in electron populations on P, F, and Cl upon complex formation do not correlate with changes in the chemical shieldings of these atoms. EOM-CCSD spin–spin coupling constants for complexes with chlorine-shared halogen bonds do not exhibit the usual dependencies on distance. 2XJ(P–F) and 2XJ(P–Cl) for complexes with chlorine-shared halogen bonds do not correlate with P–F and P–Cl distances, respectively. 1XJ(P–Cl) values for H2XP:ClF correlate best with the Cl–F distance, and approach the values of 1J(P–Cl) for the corresponding cations H2XPCl+. Values of 1XJ(P–Cl) for complexes H2XP:ClCl with chlorine-shared halogen bonds correlate with the binding energies of these complexes. 1J(F–Cl) and 1J(Cl–Cl) for complexes with chlorine-shared halogen bonds correlate linearly with the distance between P and the proximal Cl atom. In contrast, 2XJ(P–Cl) and 1XJ(P–Cl) for complexes with traditional halogen bonds exhibit more normal distance dependencies.

DOI: 10.1021/jp502443h

Neutral Alkaline-Metal and Alkaline-Earth-Metal Derivatives of Imidazole and Benzimidazole

A theoretical study of the minima and connecting transition states of the neutral complexes formed by alkaline-metal and alkaline-earth-metal derivatives of imidazolate and benzimidazolate anions has been carried out using B3LYP/6-31+G(d,p), B3LYP/6-311+G(3df,2p), and G3B3 methods. Two and three nondegenerated minima and two and four TS structures have been identified for imidazole and benzimidazole derivatives, respectively. The most stable minima of the alkaline-metal derivatives of both systems correspond to the metal interacting with the imidazole ring, whereas in the alkaline-earth-metal derivatives, the preferred minima depend on the substituent. A remarkable feature of some minima is the fact that some of the metal–aromatic interactions follow the classical π–cation pattern, even though the global structure corresponds to a neutral salt, constituting a class of noncovalent interaction of great interest in the chemistry of aromatic and heterocyclic complexes. A CSD search has confirmed that the two bonding modes, N−σ and π, are present in the solid phase. The π mode has been analyzed by comparison with other azoles.

DOI: 10.1021/jp502667k

Pnicogen-Bonded Anionic Complexes

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the pnicogen-bonded complexes H2YP:X–, for X,Y = Cl, NC, F, CCH, and CH3. Of the 36 possible complexes, only 21 are unique equilibrium structures. All substituents form (H2XPX)− complexes with symmetric X–P–X bonds. The P–A ion–molecule pnicogen bonds in these and some additional complexes have partial covalent character, while some P–A′ covalent bonds have partial ion–molecule character. A and A′ are the atoms of X and Y, respectively, which are directly bonded to P. Complexes with these types of bonds include the symmetric complexes (H2XPX)−, H2(CH3)P:F–, H2(CCH)P:F–, H2FP:NC–, H2FP:Cl–, H2FP:CN–, and H2(NC)P:Cl–. Charge transfer from A to the P–A′ σ* orbital stabilizes H2YP:X– complexes and leads to a reduction of the negative charge on X. For fixed X, the smallest negative charge occurs in the symmetric complex. Then, for a given X, the order of decreasing negative charge with respect to Y is CH3 > CCH > CN (bonded through C) > F > NC (bonded through N) > Cl, which is also the order of decreasing P–A distance. EOM-CCSD spin–spin coupling constants 1pJ(P–A) differentiate between shorter ion–molecule pnicogen bonds with partial covalent character and longer P···A ion–molecule pnicogen bonds. Similarly, coupling constants 1J(P–A′) differentiate between longer covalent P–A′ bonds with partial ion–molecule character and shorter P–A′ covalent bonds.

DOI: 10.1039/C4CC00169A

Discovery of anion–π interactions in the recognition mechanism of inorganic anions by 1,2,3-triazolium rings

A bis(triazolium)-based receptor designed for anion recognition is presented. NMR spectroscopic data indicate that one triazolium ring is acting as a hydrogen bond donor, whereas the second triazolium ring behaves as an anion–π receptor. The simultaneous presence of two noncovalent interactions allows us to achieve a highly selective binding of the hydrogenpyrophosphate anion.

DOI: 10.1021/jp500915c

Influence of Substituent Effects on the Formation of P···Cl Pnicogen Bonds or Halogen Bonds

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out in search of equilibrium structures with P···Cl pnicogen bonds or halogen bonds on the potential energy surfaces H2FP:ClY for Y = F, NC, Cl, CN, CCH, CH3, and H. Three different types of halogen-bonded complexes with traditional, chlorine-shared, and ion-pair bonds have been identified. Two different pnicogen-bonded complexes have also been found on these surfaces. The most electronegative substituents F and NC form only halogen-bonded complexes, while the most electropositive substituents CH3 and H form only pnicogen-bonded complexes. The halogen-bonded complexes involving the less electronegative groups Cl and CN are more stable than the corresponding pnicogen-bonded complexes, while the pnicogen-bonded complexes with CCH are more stable than the corresponding halogen-bonded complex. Traditional halogen-bonded complexes are stabilized by charge transfer from the P lone pair to the Cl–A σ* orbital, where A is the atom of Y directly bonded to Cl. Charge transfer from the Cl lone pair to the P–F σ* orbital stabilizes pnicogen-bonded complexes. As a result, the H2FP unit becomes positively charged in halogen-bonded complexes and negatively charged in pnicogen-bonded complexes. Spin–spin coupling constants 1XJ(P–Cl) for complexes with traditional halogen bonds increase with decreasing P–Cl distance, reach a maximum value for complexes with chlorine-shared halogen bonds, and then decrease and change sign when the bond is an ion-pair bond. 1pJ(P–Cl) coupling constants across pnicogen bonds tend to increase with decreasing P–Cl distance.

DOI: 10.1007/s00214-014-1464-y

σ–σ and σ–π pnicogen bonds in complexes H₂XP:PCX, for X = F, Cl, OH, NC, CN, CCH, CH₃, and H

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out on complexes H2XPs:PtCX, for X = F, Cl, OH, NC, CN, CCH, CH3, and H, in search of complexes stabilized by P···P pnicogen bonds. These intermolecular bonds arise when a pnicogen atom acts as a Lewis acid for complex formation. Three sets of equilibrium structures have been found on the H2XPs:PtCX potential surfaces. Conformation A complexes have P···P σ–σ pnicogen bonds, which involve the σ systems of both P atoms. Conformations B and C are stabilized by σ–π pnicogen bonds, which involve the σ system of H2XP and the π system of PCX. Binding energies of B and C complexes are similar and are greater than the binding energies of the A conformers. Charge transfer stabilizes A, B, and C conformers. In A complexes, the dominant charge transfer is from the lone pair of PCX to the antibonding σ*P–A orbital of PH2X, with A the atom of X directly bonded to P. For conformations B and C, the dominant charge transfer is from the P=C π orbital to the σ*P–A orbital of H2XP. Although the binding energies of these complexes do not correlate with the intermolecular P–P distances, both the charge-transfer energies and the equation-of-motion coupled cluster singles and doubles one-bond 31P–31P spin–spin coupling constants do correlate with the P–P distances. The largest coupling constants 1pJ(P–P) are found for complexes with conformation A, due to the nature of the σ–σ pnicogen bond and the dominance of the Fermi contact term. For a given X, 1pJ(P–P) values are ordered A > C > B.

DOI: 10.1080/00268976.2013.843034

Spontaneous proton transfers induced by beryllium bonds

Through the use of B3LYP/6-311+G(d,p) density functional theory (DFT) calculations, we have shown that when a molecule participates as a proton donor in a complex, it yields much stronger hydrogen bonds (HBs) if it participates simultaneously in beryllium bonds. This is indeed the case of the complexes formed by oxyacids such as acetic, benzoic and phosphinic acids with BeCl₂, which yield much stronger HBs with different bases than the isolated oxyacids due to a significant acidity enhancement, triggered by the charge transfer from the oxyacid to BeCl₂. More importantly, depending on the intrinsic basicity of the base acting as proton acceptor, a spontaneous proton transfer from the oxyacid to the base may occur, leading to the formation of an ion pair in the gas phase. This is indeed the case in complexes involving ammonia. For slightly weaker bases, such as trimethylphosphine, two local minima are stable: one in which the proton remains attached to the oxyacid and one in which this proton has been transferred to the trimethylphosphine, the latter being always the most stable. When a compound is able to act simultaneously as a proton donor and as a proton acceptor, its participation in a beryllium bond necessarily leads to an enhancement and a dampening of both properties, respectively. Hence, the HB in which they participate as proton donors becomes stronger, whereas the HB in which they act as proton acceptors becomes weaker. The dimers of 1H-tetrazole and the dimers of isatin nicely illustrate this finding.

DOI: 10.1021/jp411623h

Pnicogen Bonds between X=PH3 (X = O, S, NH, CH2) and Phosphorus and Nitrogen Bases

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the pnicogen bonded complexes formed between the acids O═PH3, S═PH3, HN═PH3, and H2C═PH3 and the bases NH3, NCH, N2, PH3, and PCH. All nitrogen and phosphorus bases form complexes in which the bases are lone pair electron donors. The binding energies of complexes involving the stronger bases NH3, NCH, and PH3 differentiate among the acids, but the binding energies of complexes with the weaker bases do not. These complexes are stabilized by charge transfer from the lone pair orbital of N or P to the σ*P═A orbital of X═PH3, where A is the atom of X directly bonded to P. PCH also forms complexes with the X═PH3 acids as a π electron donor to the σ*P═A orbital. The binding energies and the charge-transfer energies of the π complexes are greater than those of the complexes in which PCH is a lone pair donor. Whether the positive charge on P increases, decreases, or remains the same upon complex formation, the chemical shieldings of 31P decrease in the complexes relative to the corresponding monomers. 1pJ(P–N) and 1pJ(P–P) values correlate best with the corresponding P–N and P–P distances as a function of the nature of the base. 1J(P–A) values do not correlate with P–A distances. Rather, the absolute values of 1J(P–O), 1J(P–S), and 1J(P–N) decrease upon complexation. Decreasing 1J(P–A) values correlate linearly with increasing complex binding energies. In contrast, 1J(P–C) values increase upon complexation and correlate linearly with increasing binding energies.

DOI: 10.1021/jp412144r

Single Electron Pnicogen Bonded Complexes

A theoretical study of the complexes formed by monosubstituted phosphines (XH2P) and the methyl radical (CH3) has been carried out by means of MP2 and CCSD(T) computational methods. Two minima configurations have been obtained for each XH2P:CH3 complex. The first one shows small P–C distances and, in general, large interaction energies. It is the most stable one except in the case of the H3P:CH3 complex. The second minimum where the P–C distance is large and resembles a typical weak pnicogen bond interaction shows interaction energies between −9.8 and −3.7 kJ mol–1. A charge transfer from the unpaired electron of the methyl radical to the P–X σ* orbital is responsible for the interaction in the second minima complexes. The transition state (TS) structures that connect the two minima for each XH2P:CH3 complex have been localized and characterized.

DOI:  10.1039/c3cp55168g

Cooperativity in beryllium bonds

A theoretical study of the beryllium bonded clusters of the (iminomethyl)beryllium hydride and (iminomethyl)beryllium fluoride [HC(BeX)═NH, X = H, F] molecules has been carried out at the B3LYP/6-311++G(3df,2p) level of theory. Linear and cyclic clusters have been characterized up to the decamer. The geometric, energetic, electronic and NMR properties of the clusters clearly indicate positive cooperativity. The evolution of the molecular properties, as the size of the cluster increases, is similar to those reported in polymers held together by hydrogen bonds.

DOI: 10.1016/j.cplett.2013.07.081

Spontaneous ion-pair formation in the gas phase induced by Beryllium bonds

The changes in the structure and bonding of the hydrogen bonded complexes between hydrogen halides and ammonia, phosphine and water, when the hydrogen halides form beryllium bonds with BeCl₂ have been investigated by CCSD(T)/aug-cc-pVTZ ab initio calculations. Although the experimental evidence showed that a spontaneous proton transfer (PT) from hydrogen halides toward ammonia does not occur, and only is observed for HI when interacting with trimethylamine, we have found that a spontaneous PT does occur when the halide forms beryllium bonds with BeCl₂. The complexes so formed are the result of the interaction of Cl₂BeX⁻ with NH₄⁺, PH₄⁺, H₃O⁺, respectively.

DOI: 10.1021/jp409016q

Properties of Complexes H₂C═(X)P:PXH₂, for X = F, Cl, OH, CN, NC, CCH, H, CH₃, and BH₂: P···P Pnicogen Bonding at σ-Holes and π-Holes

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out on complexes H2C═(X)P:PXH2, for X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2. Three sets of complexes have been found on the potential surfaces. Conformation A complexes have A–P···P–A approaching linearity, with A the atom of X directly bonded to P. Conformation B complexes have A–P···P linear, but the P···P═C orientation of H2C═PX may differ significantly from linearity. Conformation C complexes are unique, since the pnicogen bond involves π-electron donation and acceptance by H2C═PX. The order of binding energies of the three conformations of H2C═(X)P:PXH2 is C > A > B, with two exceptions. Although the binding energies of conformation C complexes tend to be greater than the corresponding conformation A complexes, intermolecular distances in conformation C tend to be longer than those in conformation A. Charge transfer stabilizes H2C═(X)P:PXH2 complexes. The preferred direction of charge transfer is from H2C═PX to PXH2. In conformations A and B, charge transfer occurs from a P lone pair on one molecule to an antibonding σ* orbital on the other. However, in conformation C, charge transfer occurs from the π orbital of H2C═PX to the σ*P–A orbital of PXH2, and from the lone pair on P of PXH2 through the π-hole to the π*P═C orbital of H2C═PX. Changes in charges on P upon complexation do not correlate with changes in ³¹P chemical shieldings. Computed EOM-CCSD spin–spin coupling constants correlate with P–P distances. At each distance, the ordering of 1pJ(P–P) is A > B > C. Binding energies and spin–spin coupling constants of conformation A complexes of (PH2X)2, H2C═(X)P:PXH2, and (H2C═PX)2 with A–P···P–A approaching linearity have been compared. For complexes with the more electronegative substituents, binding energies are ordered (PH2X)2 > H2C═(X)P:PXH2 > (H2C═PX)2, while the order is reversed for complexes formed from the more electropositive substituents. A plot of ΔE(PH2X)2/ΔE(H2C═PX)2 versus ΔE[H2C═(X)P:PXH2]/ΔE(H2C═PX)2 indicates that there is a systematic relationship among the stabilities of these complexes. Complexes (PH2X)2 tend to have larger spin–spin coupling constants and shorter P–P distances than H2C═(X)P:PXH2, which in turn have larger coupling constants and shorter P–P distances than (H2C═PX)2, although there is some overlap. Complexes having similar P–P distances have similar values of1pJ(P–P).

DOI: 10.1021/ct400818v

On the Reliability of Pure and Hybrid DFT Methods for the Evaluation of Halogen, Chalcogen, and Pnicogen Bonds Involving Anionic and Neutral Electron Donors

In this article, we report a comprehensive theoretical study of halogen, chalcogen, and pnicogen bonding interactions using a large set of pure and hybrid functionals and some ab initio methods. We have observed that the pure and some hybrid functionals largely overestimate the interaction energies when the donor atom is anionic (Cl– or Br–), especially in the halogen bonding complexes. To evaluate the reliability of the different DFT (BP86, BP86-D3, BLYP, BLYP-D3, B3LYP, B97-D, B97-D3, PBE0, HSE06, APFD, and M06-2X) and ab initio (MP2, RI-MP2, and HF) methods, we have compared the binding energies and equilibrium distances to those obtained using the CCSD(T)/aug-cc-pVTZ level of theory, as reference. The addition of the latest available correction for dispersion (D3) to pure functionals is not recommended for the calculation of halogen, chalcogen, and pnicogen complexes with anions, since it further contributes to the overestimation of the binding energies. In addition, in chalcogen bonding interactions, we have studied how the hybridization of the chalcogen atom influences the interaction energies.

DOI: 10.1021/jp407097e

Pnicogen Bonded Complexes of PO2X (X = F, Cl) with Nitrogen Bases

n ab initio MP2/aug′-cc-pVTZ study has been carried out on complexes formed between PO2X (X = F and Cl) as the Lewis acids and a series of nitrogen bases ZN, including NH3, H2C═NH, NH2F, NP, NCH, NCF, NF3, and N2. Binding energies of these complexes vary from −10 to −150 kJ/mol, and P—N distances from 1.88 to 2.72 Å. Complexes ZN:PO2F have stronger P...N bonds and shorter P—N distances than the corresponding complexes ZN:PO2Cl. Charge transfer from the N lone pair through the π-hole to the P—X and P—O σ* orbitals leads to stabilization of these complexes, although charge-transfer energies can be evaluated only for complexes with binding energies less than −71 kJ/mol. Complexation of PO2X with the strongest bases leads to P···N bonds with a significant degree of covalency, and P—N distances that approach the P—N distances in the molecules PO2NC and PO2NH2. In these complexes, the PO2X molecules distort from planarity. Changes in 31P absolute chemical shieldings upon complexation do not correlate with changes in charges on P, although they do correlate with the binding energies of the complexes. EOM-CCSD spin–spin coupling constants 1pJ(P—N) are dominated by the Fermi-contact term, which is an excellent approximation to total J. 1pJ(P—N) values are small at long distances, increase as the distance decreases, but then decrease at short P—N distances. At the shortest distances, values of1pJ(P—N) approach 1J(P—N) for the molecules PO2NC and PO2NH2.

DOI: 10.1021/jp407339v

Non-Covalent Interactions: Complexes of Guanidinium with DNA and RNA Nucleobases

Considering that guanidine-based derivatives are good DNA minor groove binders, we have theoretically studied, using the Polarizable Continuum model mimicking water solvation, the complexes formed by the biologically relevant guanidinium cation and the DNA and RNA nucleobases (adenine, guanine, cytosine, thymine, and uracil). The interactions established within these complexes both by hydrogen bonds and by cation−π interactions have been analyzed by means of the Atoms in Molecules and Natural Bond Orbital approaches. Moreover, maps of electron density difference have been produced to understand the cation−π complexes. Finally, the NICS and three-dimensional NICS maps of the cation−π complexes have been studied to understand the effect of the guanidinium cation on the aromaticity of the nucleobases.

DOI: 10.1016/j.crci.2013.05.016

Intermolecular spin–spin coupling constants between ³¹P atoms

DOI: 10.1007/s00894-012-1682-y

Enhancing and modulating the intrinsic acidity of imidazole and pyrazole through beryllium bonds

The structure and electronic properties of the complexes formed by the interaction of imidazole and pyrazole with different BeXH(BeX2) (X = H, Me, F, Cl) derivatives have been investigated via B3LYP/6−311+G(3df,2p)//B3LYP/6−31+G(d,p) calculations. The formation of these azole:BeXH(BeX2) complexes is accompanied by a dramatic enhancement of the intrinsic acidity of the azole, as the deprotonated azole is much more stable after the aforementioned interaction. Most importantly, the increase in acidity is so large that the azole:BeXH or azole:BeX2 complexes behave as NH acids, which are stronger than typical oxyacids such as phosphoric acid and oxalic acid. Interestingly, the increase in acidity can be tuned through appropriate selection of the substituents attached to the Be atom, permitting us to modulate the electron-accepting ability of the BeXH or BeX2 molecule.

DOI: 10.1021/jp4063109

Characterizing Complexes with Pnicogen Bonds Involving sp2 Hybridized Phosphorus Atoms: (H2C═PX)2 with X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2

Ab initio MP2/aug′-cc-pVTZ searches of the potential surfaces of (H2C═PX)2 complexes, with X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2, have been carried out to identify and characterize the properties of complexes with P···P pnicogen bonds. All (H2C═PX)2 form equilibrium conformation A dimers with C2h symmetry in which A–P···P–A approaches a linear alignment, with A the atom of X directly bonded to P. Conformation A dimers containing the more electronegative substituents are stabilized by a P···P pnicogen bond, have shorter P–P distances, and have binding energies which correlate with the P–P distance. Dimers stabilized by a P···P pnicogen bond and two P···Hb interactions consist of those with the more electropositive substituents, have shorter P–Hb distances, and have binding energies which are too high for their P–P distances. Conformation A complexes with P···Hbinteractions in addition to the P···P bond are more stable than the corresponding (PH2X)2complexes, while with only one exception, complexes stabilized by only a P···P bond are less stable than the corresponding (PH2X)2 complexes. In the region of the potential surfaces with C–P···P–C approaching linearity (conformation B), the only planar equilibrium complex is (H2C═POH)2, which is stabilized primarily by two O–H···P hydrogen bonds. The remaining (H2C═PX)2 complexes are not stabilized by pnicogen bonds, but by π interactions between the two H2C═PX monomers which are in parallel planes. When A–P···P–C approaches linearity, two types of equilibrium structures with P···P bonds exist. Of the conformation C dimers, (H2C═POH)2 is planar and the most stable, with a P···P pnicogen bond and an O–H···P hydrogen bond. (H2C═PH)2 and (H2C═PCH3)2 are also planar, and stabilized by a P···P pnicogen bond and a P···Hb interaction. The absence of a P···Hb interaction results in nonplanar C′ conformations with structures in which the monomers essentially retain their symmetry plane, but the plane of one molecule is rotated about the P···P bond relative to the other. C and C′ dimers are less stable than the corresponding A dimers, except for (H2C═PCH3)2. 31P chemical shielding patterns are consistent with the changing nature of the interactions which stabilize (H2C═PX)2 complexes. EOM-CCSD 31P–31P spin–spin coupling constants increase quadratically as the P–P distance decreases.

DOI: 10.1039/c3cp52312h

Orthogonal interactions between nitryl derivatives and electron donors: pnictogen bonds

Pnictogen complexes between nitryl derivatives (NO₂X, X = CN, F, Cl, Br, NO₂, OH, CCH, and C₂H₃) and molecules acting as Lewis bases (H₂O, H₃N, CO, HCN, HNC and HCCH) have been obtained at the MP2/aug-cc-pVTZ computational level. A total of 53 minima have been located. Their energy, geometry, DFT-SAPT energy terms, electronic properties (NBO, AIM, ELF, and NCI) and NMR shieldings have been calculated and analyzed. Finally, a search in the CSD database has been carried out, showing a large number of similar interactions in crystallographic structures.

DOI: 10.1021/jp405211p

Substituent Effects on the Cooperativity of Halogen Bonding

DFT calculations (B97-1) with the 6-31+G(d,p)-LanL2DZdp basis set were used to analyze the intermolecular interactions in 4-Z-Py···XCN···XCN triads (Z = H, F, OH, OCH3, CH3, NH2, NO2, and CN; Py = pyridine; and X = Cl and Br) that are connected by halogen-bond interactions. To understand the properties of the systems better, the corresponding dyads are also studied. Particular attention is given to parameters such as cooperative energy. All complexes show cooperative energy ranging from −1.39 to −3.46 kJ mol–1 and −2.61 to −5.84 kJ mol–1 for X = Cl and Br, respectively. We show that the effect of the substituents on the title interactions strongly depends on the nature of the substituents (Z). Thus, the electron-donor and electron-acceptor substituents increase and decrease the stability of complexes, respectively. The electronic properties of the complexes have been analyzed using molecular electrostatic potential (MEP) and minimum average local ionization energy, and the parameters were derived from the atoms in molecules (AIM) and natural bond orbital (NBO) methodologies.

DOI: 10.1524/zpch.2013.0367

Ab Initio Study of Cooperative Effects in Complexes X:HBO:Z, with X, Z=LiH, HNC, HF, HCN, HCl, ClF, and HBO: Structures, Binding Energies, and Spin-Spin Coupling Constants across Intermolecular Bonds

A systematic ab initio investigation has been carried out to determine the structures, binding energies, and spin-spin coupling constants of ternary complexes X:HBO:Z for X, Z=LiH, HNC, HF, HCN, HCl, ClF, and HBO. All complexes X:HBO:Z are linear with C∞v symmetry, except for HCl:HBO:Z and ClF:HBO:Z which have Cs symmetry, thereby reflecting the structures of the corresponding X:HBO and HBO:Z complexes. Cooperative effects on energies are synergistic in all ternary complexes. The enhanced binding energies of complexes X:HBO:Z correlate with the binding energies of the X:HBO and HBO:Z complexes. Coupling constants ¹J(B-H) and ^2hJ(B-A) across B-H···A hydrogen bonds correlate with the B-A distance, and exhibit synergistic effects due to the presence of Z. ^1hJ(H-A) indicates that these bonds have little proton-shared character. Coupling constants across D-H···O hydrogen bonds, H-Li···O lithium bonds, and F-Cl···O halogen bonds are also sensitive to the synergistic effects arising from the presence of X. D-H···O hydrogen bonds in ternary complexes are traditional (normal) hydrogen bonds.

DOI: 10.1016/j.molstruc.2013.04.069

Resonance assisted hydrogen bonds in open-chain and cyclic structures of malonaldehyde enol: A theoretical study

In 1989 Gilli, Bellucci, Ferretti and Bertolasi introduced the notion of Resonance Assisted Hydrogen Bonding (RAHB) one of the most fruitful concepts in structural chemistry. After reviewing our previous contributions to this topic, the present work analyzes theoretically this concept especially in non-cyclic structures. Geometries, electron densities and Laplacian at the bond critical points, cooperativity through many body interaction energies, deformation energies as well as NMR properties (chemical shifts and ^2hJ_OO coupling constants) are used for the discussion.

DOI: 10.1021/jp403651h

Pnicogen-Bonded Cyclic Trimers (PH₂X)₃ with X = F, Cl, OH, NC, CN, CH₃, H, and BH₂

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to determine the structures and binding energies of cyclic trimers (PH2X)3 with X = F, Cl, OH, NC, CN, CH3, H, and BH2. Except for [PH2(CH3)]3, these complexes have C3h symmetry and binding energies between −17 and −63 kJ mol–1. Many-body interaction energy analyses indicate that the two-body terms are dominant, accounting for 97–103% of the total binding energy. Except for the trimer [PH2(OH)]3, the three-body terms are stabilizing. Charge transfer from the lone pair on one P atom to an antibonding σ* orbital of the P atom adjacent to the lone pair plays a very significant role in stabilization. The charge-transfer energies correlate linearly with the trimer binding energies. NBO, AIM, and ELF analyses have been used to characterize bonds, lone pairs, and the degree of covalency of the P···P pnicogen bonds. The NMR properties of chemical shielding and 31P–31P coupling constants have also been evaluated. Although the 31P chemical shieldings in the five most strongly bound trimers increase relative to the corresponding isolated monomers, there is no correlation between the chemical shieldings and the charges on the P atoms. EOM-CCSD 31P–31P spin–spin coupling constants computed for four (PH2X)3 trimers fit nicely onto a plot of 1pJ(P–P) versus the P–P distance for (PH2X)2dimers. A coupling constant versus distance plot for the four trimers has a second-order trendline which has been used to predict the values of 1pJ(P–P) for the remaining trimers.

DOI: 10.1002/cphc.201300145

Intramolecular Pnicogen Interactions in PHF(CH2)nPHF (n=2–6) Systems

A computational study of the intramolecular pnicogen bond in PHF(CH₂)_nPHF (n=2–6) systems was carried out. For each compound, two different conformations, (R,R) and (R,S), were considered on the basis of the chirality of the phosphine groups. The characteristics of the closed conformers, in which the pnicogen interaction occurs, were compared with those of the extended conformer. In several cases, the closed conformations are more stable than the extended conformations. The calculated interaction energies of the pnicogen contact, by means of isodesmic reactions, provide values between −3.4 and −26.0 kJ mol⁻¹. Atoms in molecules and electron localization function analysis of the electron density showed that the systems in the closed conformations with short P⋅⋅⋅P distances have a partial covalent character in this interaction. The calculated absolute chemical shieldings of the P atoms showed an exponential relationship with the P⋅⋅⋅P distance. In addition, a search in the Cambridge crystallographic database was carried out to detect those compounds with a potential intramolecular pnicogen bond in the solid phase.

DOI: 10.1002/poc.3099

Solvent effects on guanidinium-anion interactions and the problem of guanidinium Y-aromaticity

We have calculated the complexes formed by guanidine/guanidinium and HCl/Cl⁻, HNO₃/NO₃⁻ and H₂SO₄/HSO₄⁻ both in the gas and aqueous Polarizable Continuum Model (PCM) phase to understand the effect that solvation has on their interaction energies. In the gas phase, the cation–anion complexes are much more stable than the rest; however, when PCM-water is considered, this energetic difference is not as large due to the extra stabilization that the ions suffer when in aqueous solution. All the complexes were analyzed in terms of their AIM and NBO properties. In all cases, water solvation seems to “dampen” those properties observed in the gas phase. The values of Nucleus Independent Chemical Shift (NICS)(1) and NICS(2) indicate a huge influence of the proximity of the carbon atom for short distances; thus, the 3D NICS values on the van der Waal isosurfaces have been used to evaluate the possible Y-aromaticity of the guanidinium system. The isosurface in this system is more similar to cyclohexane than to benzene as indication of poor aromaticity. 

DOI: 10.1021/jp4016933

Complexes between Dihydrogen and Amine, Phosphine, and Arsine Derivatives. Hydrogen Bond versus Pnictogen Interaction

A theoretical study of the complexes between dihydrogen, H₂, and a series of amine, phosphine, and arsine derivatives (ZH₃ and ZH₂X, with Z = N, P, or As and X = F, Cl, CN, or CH₃) has been carried out using ab initio methods (MP2/aug-cc-pVTZ). Three energetic minima configurations have been characterized for each case with the H₂ molecule in the proximity of the pnictogen atom (Z). In configuration A, the σ-electrons of H₂ interact with σ-hole region of the pnictogen atom generated by the of X–Z bond. These complexes can be ascribed as pnictogen bonded. In configuration C, the lone electron pair of Z acts as the Lewis base, and H₂ plays the role of the Lewis acid. Finally, configuration B presents a variety of noncovalent interactions depending on the binary complex considered. The atoms-in-molecules theory (AIM), natural bond orbitals (NBO) method as well as the density functional theory–symmetry adapted perturbation theory (DFT-SAPT) approach were used in this study to deepen the nature of the interactions considered.

DOI: 10.1021/jp401480y

Phosphorus As a Simultaneous Electron-Pair Acceptor in Intermolecular P···N Pnicogen Bonds and Electron-Pair Donor to Lewis Acids

Ab initio MP2/aug’-cc-pVTZ calculations have been performed to investigate the structures and energies of binary complexes LA:PH₂F and LA:PH₃ and of ternary complexes LA:H₂FP:NFH₂ and LA:H₃P:NH₃ in which the pnicogen-bonded P atom also acts as an electron-pair donor to a Lewis acid (LA), for LA = BH₃, NCH, ClH, FH, FCl, and HLi. Hydrogen bonds, halogen bonds, and dative covalent bonds are found at P in some cases, depending on the nature of the Lewis acid. HLi forms a lithium bond with P only in the binary complex HLi:PH₃. The binding energies of ternary complexes exhibit a classical synergistic effect, although the computed cooperativity may be overestimated due to neglect of the interaction of the Lewis acid with NH₂F or NH₃ in some cases. The hydrogen-bonding Lewis acids appear to have little effect on the strength of the P···N bond, while the remaining Lewis acids strengthen the pnicogen bond. ³¹P absolute chemical shieldings increase in LA:H₂FP:NFH₂complexes relative to the corresponding LA:PH₂F complexes as the positive charge on P decreases, while chemical shieldings decrease in LA:H₃P:NH₃ relative to the corresponding LA:PH₃ complexes as the positive charge increases. Absolute values of ^1pJ(P–N) spin–spin coupling constants in complexes LA:H₂FP:NFH₂ decrease as the P–N distance decreases. It appears that this behavior is associated with the presence of a second intermolecular interaction, whether electron-donation by P or hydrogen bond formation at P–F.

DOI: 10.1039/C2CE26786A

Linear free energy relationships in halogen bonds

Four models of halogen bonds were used to quantify this bond using the DFT B97D/6-311+G(d) computational level: para-substituted iodobenzenes, para- and meta-substituted bromobenzenes complexed with three simple Lewis bases (NH₃, NCH and CNH), 1-bromo-4-substituted-bicyclo[2.2.2]octanes with NH₃ and 3- and 4-substituted pyridines complexed with BrCl and BrF. In addition, the combination of the para-substituted bromobenzenes with the 4-substituted pyridines has been studied. A total of 459 complexes have been optimized and are discussed in the present article. The energetic and geometric results have been analyzed based on the properties of the substituents and the isolated molecules involved in the interaction. The Hammett–Taft parameters provide reasonable correlations with the interaction energies. However, excellent correlations are obtained in all the cases when the electrostatic properties of the two molecules involved in the interaction are considered (R² > 0.99).

DOI:10.1007/s11224-012-0099-7

Ab initio study of water clustering in the presence of a methyl radical

The geometrical structure and binding energy of small clusters of methyl radical and water molecules (up to five water molecules) in gas phase and water media have been investigated at the MP2 level of theory using 6-311++G(2df,2p) basis set. The complexes characterized contain OH···O, CH···O, and OH···C attractive interactions with stabilization energies in the range 6–143 kJ mol⁻¹. The solvent has an enhancing influence on the stabilities of studied clusters. The atoms in molecules theory were also applied to explain the nature of the complexes. The interaction energies have been partitioned with the natural energy decomposition analysis showing that the most important attractive term corresponds to the charge transfer one.

DOI: 10.1021/jp3100816

Exploring (NH₂F)₂, H₂FP:NFH₂, and (PH₂F)₂ Potential Surfaces:Hydrogen Bonds or Pnicogen Bonds?

An ab initio MP2/aug’-cc-pVTZ study has been carried out to identify local minima on the (NH₂F)₂, H₂FP:NFH₂, and (PH₂F)₂ potential surfaces, to characterize the types of interactions which stabilize the complexes found at these minima, and to evaluate their binding energies. With one exception, (NH₂F)₂ complexes are stabilized by N–H···N or N–H···F hydrogen bonds. Only one complex, that with the smallest binding energy, has a pnicogen N···N bond. In contrast, (PH₂F)₂ complexes are stabilized by P···P or P···F pnicogen bonds or by an antiparallel alignment of the dipole moment vectors of the two monomers, but not by hydrogen bonds. The most stable complex has an F–P···P–F alignment which approaches linearity. Both hydrogen-bonded and pnicogen-bonded complexes exist on the H₂FP:NFH₂surface, with the most stable being the pnicogen-bonded complex with F–P···N–F approaching a linear arrangement. Charge transfer transitions from a lone pair on a P, N, or F atom in one molecule to an antibonding σ* orbital of the other stabilize these complexes. These transitions are most important for complexes with pnicogen bonds. Although net charge transfer occurs in complexes in which the two monomers are inequivalent, charges on N and P do not correlate with N and P absolute chemical shieldings. Rather, these shieldings also reflect charge distributions and overall bonding patterns. EOM-CCSD two-bond spin–spin coupling constants ^2hJ(X–Y) across X–H···Y hydrogen bonds tend to be small, due in part to the nonlinearity of many of the hydrogen bonds. ^1pJ values across a particular kind of pnicogen bond are relatively large and vary significantly but do not correlate with corresponding distances.

DOI:10.1016/j.cplett.2012.10.073

Tracing environment effects that influence the stability of anion–anion complexes: The case of phosphate–phosphate interactions

The effect of the environment on the stability of the (H₃PO₄), (H₂PO_4^-) and (HPO_4^2-)₂ hydrogen bonded dimers has been explored by the topological analyses of the theoretical electron density and the electrostatic potential. The environment has little effect on the hydrogen-bonding interaction, while it induces a significant one on the Coulombic component of the dimer. The interaction energy is represented in terms of hydrogen-bond and non-hydrogen-bond contributions, being only the latter affected by the charge or the environment. While the non-hydrogen bond contribution dominates the interaction energy in the gas phase, it becomes balanced in a polarizable environment.

DOI: 10.1002/poc.3017

A theoretical study of hemiacetal formation from the reaction of methanol with derivatives of CX₃CHO (X = H, F, Cl, Br and I)

A theoretical study of the hemiacetal formation reaction between methanol and CX₃CHO (X = H, F, Cl, Br, and I) has been carried out using density functional theory and Becke, three-parameter, Lee–Yang–Parr/6-311++G(d,p) computational methods. The stationary points of the reaction between the isolated molecules and the reaction catalyzed by an additional methanol molecule have been characterized. Because the final products present a stereogenic center, the potential autocatalysis of the reaction has been examined and also the possibility of spontaneous generation of chirality when the hemiacetal molecules are involved in the transition state structure. High barriers are found in the reaction between the isolated molecules that are reduced by the assistance of an additional molecule (methanol or hemiacetal product). The reactions catalyzed by the hemiacetal products show higher barriers than the one catalyzed by methanol. 

DOI:10.1016/j.comptc.2012.07.002

Analysis of the interactions between difluoroacetylene and one or two hydrogen fluoride molecules based on calculated spin–spin coupling constants

A theoretical study of FCCF:(HF)_n complexes, with n = 1 and 2, has been carried out by means of ab initio computational methods. Two types of complexes are formed: those with FH⋯π interactions and those with FH⋯FC hydrogen bonds. The indirect spin–spin coupling constants have been calculated at the CCSD/aug-cc-pVTZ-J computational level. Special attention has been paid to the dependence of the different intramolecular coupling constants in FCCF on the distance between the coupled nuclei and the presence or absence of the hydrogen fluoride molecule. The sensitivity shown by these coupling constants to the presence of hydrogen fluoride is quite notorious.

DOI: 10.1021/jp300763d

Structures, Binding Energies, and Spin-Spin Coupling Constants of Geometric Isomers of Pnicogen Homodimers (PHFX)2, X = F, Cl, CN, CH3, NC

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to determine the structures and binding energies of homodimers (PHFX)₂ for X = F, Cl, CN, CH₃, and NC. Geometric isomers of these complexes with C_i symmetry exist, which are differentiated in terms of the nature of the atoms (F–P···P–F, H–P···P–H, or A–P···P–A, with A being the atom of X directly bonded to P), which approach a nearly linear alignment. Of these, isomers having F–P···P–F linear are the most stable. Binding energies, intermolecular distances, and EOM-CCSD spin–spin coupling constants are sensitive to both the nature of X and the atoms that assume the linear alignment.

DOI: 10.1002/cphc.201200068

Electrostatics at the Origin of the Stability of Phosphate-Phosphate Complexes Locked by Hydrogen Bonds

Ab initio calculations reveal that hydrogen bonds can lock phosphates into stable gas-phase complexes, showing that hydrogen bonding can overcome anion–anion repulsion. These complexes present a large energetic barrier of dissociation (see picture). The stability of the complexes can be explained in terms of the electrostatic interaction in the hydrogen-bond region.

DOI: 10.1021/jp211451y

FCl:PCX Complexes: Old and New Types of Halogen Bonds

MP2/aug′-cc-pVTZ calculations have been performed to investigate the halogen-bonded complexes FCl:PCX, for X = NC, CN, F, H, CCH, CCF, CH₃, Li, and Na. Although stable complexes with a F–Cl···P halogen bond exist that form through the lone pair at P (configuration I), except for FCl:PCCN, the more stable complexes are those in which FCl interacts with the C≡P triple bond through a perturbed π system (configuration II). In complexes I, the nature of the halogen bond changes from traditional to chlorine-shared and the interaction energies increase, as the electron-donating ability of X increases. The anionic complex FCl:PC^– has a chlorine-transferred halogen bond. SAPT analyses indicate that configuration I complexes with traditional halogen bonds are stabilized primarily by the dispersion interaction. The electrostatic interaction is the most important for configuration I complexes with chlorine-shared halogen bonds and for configuration II complexes except for FCl:PCNa for which the induction term is most important. The F–Cl stretching frequency is red-shifted upon complexation. EOM-CCSD/(qzp,qz2p) spin–spin coupling constants have been obtained for all FCl:PCX complexes with configuration I. ¹J(F–Cl) decreases upon complexation. ^2XJ(F–P) values are quadratically dependent upon the F–P distance and are very sensitive to halogen-bond type. ^1XJ(Cl–P) tends to increase as the Cl–P distance decreases but then decreases dramatically in the chlorine-transferred complex FCl:PC^– as the Cl–P interaction approaches that of a covalent Cl–P bond. Values of ¹J(F–Cl) for configuration II are reduced relative to configuration I, reflecting the longer F–Cl distances in II compared to those of the neutral complexes of I. Although the F–P and Cl–P distances in configuration II complexes are shorter than these distances in the corresponding configuration I complexes,^2XJ(F–P) and ^1XJ(Cl–P) values are significantly reduced, indicating that coupling through the perturbed C–P π bond is less efficient. The nature of F–P coupling for configuration II is also significantly different, as evidenced by the relative importance of PSO, FC, and SD components.

DOI: 10.1002/cphc.201100830

Intermolecular Weak Interactions in HTeXH Dimers (X=O, S, Se, Te): Hydrogen Bonds, Chalcogen-Chalcogen Contacts and Chiral Discrimination

A theoretical study of the HTeXH (X=O, S, Se and Te) monomers and homodimers was carried out by means of second-order Møller-Plesset perturbation theory (MP2) computational methods. In the case of monomers, the isomerization energy from HTeXH to H₂Te=X and H₂X=Te (X=O, S, Se, and Te) and the rotational transition-state barriers were obtained. Due to the chiral nature of these compounds, homo and heterochiral dimers were found. The electron density of the complexes was characterized with the atoms-in-molecules (AIM) methodology, finding a large variety of interactions. The charge transfer within the dimers was analyzed by means of natural bond orbitals (NBO). The density functional theory-symmetry adapted perturbation theory (DFT-SAPT) method was used to compute the components of the interaction energies. Hydrogen bonds and chalcogen–chalcogen interactions were characterized and their influence analyzed concerning the stability and chiral discrimination of the dimers.

DOI: 10.1007/s11224-012-9947-8

A theoretical reappraisal of the cyclol hypothesis

Theoretical calculations at the B3LYP/6-31G(d) level have been carried out on the isomerization of cyclic-tri-glycine into the corresponding tri-cyclol. The results confirmed that the cyclol hypothesis was untenable both from a thermodynamic as well as from a kinetic point of view.

 DOI: 10.1007/s11224-011-9931-8

A theoretical study of 1:1 and 1:2 complexes of acetylene with nitrosyl hydride

Ab initio calculations at MP2 computational level using aug-cc-pVTZ basis set were used to analyze the interactions between 1:1 and 1:2 complexes of acetylene and nitrosyl hydride. The structures obtained have been analyzed with the atoms in molecules and the density functional theory–symmetry adapted perturbation theory methodologies. Four minima were located on the potential energy surface of the 1:1 complex. Twenty-four different structures have been obtained for the 1:2 complexes. Five types of interactions are observed, CH···O, CH···N, NH···π hydrogen bonds and orthogonal interactions between the π clouds of triple bond, or the lone pair of oxygen with the electron-deficient region of the nitrogen atom. Stabilization energies of the 1:1 and 1:2 clusters including basis set superposition error and ZPE are in the range 3–8 and 6–17 kJ mol⁻¹ at MP2/aug-cc-pVTZ computational level, respectively. Blue shift of NH bond upon complex formation in the ranges between 18–30 and 20–96 cm⁻¹ is predicted for 1:1 and 1:2 clusters, respectively. The total nonadditive energy in the 1:2 cluster, calculated as the sum of the supermolecular nonadditive MP2 energy and the three-body dispersion energy, presents values between −1.48 and 1.20 kJ mol⁻¹.

DOI: 10.1016/j.cplett.2012.04.039

Homo- and heterochiral dimers (PHFX)2, X = Cl, CN, CH3, NC: To what extent do they differ?

Ab initio MP2/aug’-cc-pVTZ calculations have been performed to determine if intermolecular P–P distances, Z–P–P angles, binding energies, ³¹P chemical shieldings, or EOM–CCSD spin–spin coupling constants can differentiate between corresponding C₂ (homochiral) and C_i (heterochiral) dimers (PHFX)₂, X = Cl, CN, CH₃, NC. With one exception, C_i isomers have shorter P–P distances than corresponding C₂ isomers. Neither binding energies, Z–P–P angles, chemical shieldings, nor spin–spin coupling constants ^1pJ(P–P) exhibit patterns which distinguish between corresponding C₂ and C_i isomers. ^1pJ(P–P) values correlate linearly with P–P distances, so that experimental values of ^1pJ(P–P) could be used to extract intermolecular P–P distances.

DOI: 10.1021/jp300540z

Competition and Interplay between sigma-Hole and pi-Hole Interactions: A Computational Study of 1:1 and 1:2 Complexes of Nitryl Halides (O2NX) with Ammonia

 Quantum calculations at the MP2 cc-pVTZ, MP2 aug-cc-pVTZ, and CCSD(T) cc-pVTZ levels have been used to examine 1:1 and 1:2 complexes between O₂NX (X = Cl, Br, and I) with NH₃. The 1:1 complexes can easily be differentiated using the stretching frequency of the N–X bond. Thus, those complexes with σ-hole interaction show a blue shift of the N–X bond stretching whereas a red shift is observed in the complexes along the π-hole. The SAPT-DFT methodology has been used to gain insight on the source of the interaction energy. In the 1:2 complexes, the cooperative and diminutive energetic effects have been analyzed using the many-body interaction energies. The nature of the interactions has been characterized with the atoms in molecules (AIM) and natural bond orbital (NBO) methodologies. Stabilization energies of 1:1 and 1:2 complexes including the variation of the zero point vibrational energy (ΔZPVE) are in the ranges 7–26 and 14–46 kJ mol^–1, respectively.

DOI: 10.1016/j.comptc.2012.04.007

Electron density shift description of non-bonding intramolecular interactions

A new methodology is described for the study of the electron density shift in intramolecular interactions. The methodology has been tested in an intermolecular complex and compared to the electron density shift obtained as the difference between the complex and the isolated monomers. The molecular fragmentation procedures and its application to hydrogen bonds, chalcogen–chalcogen interactions, nitrogen–boron interactions, dihydrogen interactions and silicon–nitrogen interactions are described. A careful selection of the fragmentation scheme is necessary in order to describe correctly the electron density shift in the intramolecular interactions. For this reason, different orders of fragmentation have been studied and analyzed pointing out the problems and limitations which are inherent to the methodology. It has been found that this methodology is a new tool which provides a good qualitative description of the electron density shift within the interacting region between two or more contacts, in both inter and intramolecular contacts with a reasonable low computational cost.

DOI: 10.1039/C2CP40949F

Weak interactions between hypohalous acids and dimethylchalcogens

The complexes formed between dimethylchalcogens X(CH₃)₂ (X = S, Se, and Te) and hypohalous acids YOH (Y = F, Cl, Br, and I) have been studied at the MP2/aug'-cc-pVTZ computational level, five minima structures being located. Two of them correspond to hydrogen bonds (HB), another two to halogen bonds (XB) with the chalcogen acting as an electron donor, the last one showing a C–H⋅⋅⋅O contact. The most stable complexes of IOH and BrOH acids present halogen⋅⋅⋅chalcogen interactions with interaction energies, E_i, up to −49 kJ mol⁻¹. In the case of the ClOH and FOH molecules, the hydrogen bonded complexes are more stable with interaction energies between −27 and −34 kJ mol⁻¹. Linear correlations between the molecular electrostatic potential (MEP) stationary points at the van der Waals surface and the interaction energy have been found. The contribution of the different energy terms to the total interaction energy was analyzed by means of the DFT–SAPT theory finding that the electrostatic attractive term is dominant in the complexes with HB and XB, excepting a few cases in which the dispersion and induction terms become more important than the electrostatic one.

DOI: 10.1021/ct300243b

Modulating the Strength of Hydrogen Bonds through Beryllium Bonds

The mutual influence between beryllium bonds and inter- or intramolecular hydrogen bonds (HBs) has been investigated at the B3LYP/6-311++G(3df,2p) level of theory, using the complexes between imidazole dimer and malonaldehyde with BeH₂ and BeF₂ as suitable model systems. Imidazole and its dimer form very strong beryllium bonds with both BeH₂ and BeF₂, accompanied by a significant geometry distortion of the Lewis acid. More importantly, we have found a clear cooperativity between these two noncovalent interactions, since the intermolecular HB between the two imidazole molecules in the dimer–BeX₂ complex becomes much stronger than in the isolated dimer, whereas the beryllium bond becomes also stronger in the dimer–BeX₂ complex, with respect to that found in the imidazole–BeX₂ complex. The effects of beryllium bonds are also dramatic on the strength of intramolecular HBs. Depending on to which center the BeX₂ is attached, the intramolecular HB becomes much stronger or much weaker. The first situation is found when the beryllium derivative is attached to the HB donor, whereas the second occurs if it is attached to the HB acceptor. The first effect can be so strong as to produce a spontaneous proton transfer, as it is actually the case of the malonaldehyde–BeF₂ complex.

DOI: 10.1021/ct300399y

Influence of Hydrogen Bonds on the P...P Pnicogen Bond

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the influence of F–H···F hydrogen bonds on the P···P pnicogen bond in complexes nFH:(PH₂F)₂ for n = 1–3. The formation of F–H···F hydrogen bonds leads to a shortening of the P–P distance, a lengthening of the P–F distance involved in the hydrogen bond, a strengthening of the P···P interaction, and changes in atomic populations, NMR ³¹P chemical shieldings, and ^1pJ(P–P) coupling constants. The magnitude of these changes depends on the number of FH molecules and their positions in the complex and are relatively modest except for complexes 2FH:(PH₂F)₂and 3FH:(PH₂F)₂ that have all FH molecules hydrogen bonded to the same F-atom. For these two complexes, ^1pJ(P–P) decreases as the P–P distance decreases and approaches the value of ¹J(P–P) for P₂H₄. The dramatic changes in these two complexes reflect the changing nature of the hydrogen bonds and the pnicogen bond. Thus, the complex 3FH:(PH₂F)₂ acquires ion-pair character represented as [3(FH)F^–:(H₂P–PH₂F)⁺], and the P···P pnicogen bond acquires significant covalent character. These changes are observed to a lesser extent in 2FH:(PH₂F)₂

DOI: 10.1016/j.comptc.2012.06.019

Thermodynamic and hydrogen-bond basicity of phosphine oxides: Effect of the ring strain

A theoretical study of acidity and hydrogen bond acceptor properties of tetrahedric phosphine oxide derivatives have been carried out by means of MP2 computational methods. The results obtained for the mentioned complexes have been compared with the analogous ones of trimethylphosphine oxide. The strain decreases the complexation energy with metallic atoms as well as the thermodynamic and hydrogen bond acceptor (HBA) ability of the tetrahedric derivatives.

DOI: 10.1016/j.cplett.2012.04.034

Variations in the structures and binding energies of binary complexes with HBO

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to determine the structures and binding energies of binary complexes formed by HBO with a series of small molecules A. Three different types of structures have been identified, which depend on the nature of A. In one structure A:HBO, HBO acts as a weak proton donor. In the second HBO:A, HBO is a relatively strong base. The third type of complex A||HBO has HBO and A in an approximately parallel arrangement. The dipole moment of A influences both the type of complex formed and its binding energy.

DOI: 10.1021/jp307083g

Interplay of F-H...F Hydrogen Bonds and P...N Pnicogen Bonds

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the influence of F–H···F hydrogen bonds on the P···N pnicogen bond in complexes nFH:(H₂FP:NFH₂) for n = 1–2, and a selected complex with n = 3. The NBO analysis indicates that the N(lp) → P–Fσ* charge-transfer transition has a much greater stabilizing effect than the P(lp) → N–Fσ* transition. When hydrogen bonding occurs at P–F, charge transfer associated with the pnicogen bond and the hydrogen bond are in the same direction but are in opposite directions when hydrogen bonding occurs at N–F. As a result, the formation of F–H···F hydrogen bonds at P–F leads to shorter P···N distances, increased strength of P···N bonds, and synergistic energetic effects; hydrogen bonding at N–F has opposite effects. ³¹P and ¹⁵N chemical shieldings do not correlate with charges on P and N, respectively, but ³¹P shieldings correlate quadratically with the P–N distance. ^1pJ(P–N) coupling constants do not correlate with the intermolecular P–N distance. However, when hydrogen bonding occurs only at P–F, ^1pJ(P–N) decreases in absolute value as the P–N distance decreases, thereby approaching ¹J(P–N) for H₂P–NH₂. However, the P···N bond in 3FH:(H₂FP:NFH₂) has little covalent character, unlike the P···P bond in the corresponding complex 3FH:(PH₂F)₂.

DOI: 10.1080/00268976.2011.621458

Theoretical study of the HXYH dimers (X, Y = O, S, Se). Hydrogen bonding and chalcogen-chalcogen interactions

A theoretical study of the HXYH (X, Y = O, S and Se) monomers and dimers has been carried out by means of MP2 computational methods. For the monomers, isomerization (H₂X=Y//HXYH) and rotational transition state barriers have been calculated. Additionally, the molecular electrostatic potential of the isolated monomers has also been analysed. Due to the chiral nature of these compounds, homo and heterochiral dimers have been explored. The number of minima found for the dimers range between 13 and 22. The electron density of the complexes has been characterized with the Atoms in Molecules (AIM) methodology finding a large variety of interactions. The DFT-SAPT method has been used to analyse the components of the interaction energies. Concerning chalcogen–chalcogen interactions, although the most stable minima are formed through hydrogen bonds (especially if OH groups are present in the molecules) as the size of the atoms involved in the interaction increase, the chalcogen–chalcogen contacts become more important.

DOI: 10.1021/jp205300c

Simultaneous Interactions of Anions and Cations with Cyclohexane and Adamantane: Aliphatic Cyclic Hydrocarbons as Charge Insulators

With ab initio MP2 computational methods, a theoretical study has been carried out to characterize the interaction between aliphatic cyclic hydrocarbons, as models of molecular hydrocarbon monolayers, with cations (Li⁺, Na⁺, and K⁺), anions (F^–, Cl^–, and Br^–), and both simultaneously in opposite faces of the hydrocarbons. In addition, the energetic barrier for the cation crossing through the hydrocarbon ring has been calculated. The hydrocarbons chosen for this study are cyclohexane (C₆H₁₂) and adamantane (C₁₀H₁₆). The energies obtained for the M⁺:hydrocarbon:X^– complexes indicate positive cooperativity in the cases where the hydrocarbon is cyclohexane while diminutive effects are found in the adamantane complexes. The density functional theory–symmetry adapted perturbation theory analysis of the interaction energies shows that the most important term in the complexes with cations is the induction, while in the complexes with anion and with cations and anions simultaneously the most important term is the repulsion-exchange one. The electron density of the complexes has been analyzed using the atoms in molecules methodology and provides some insight to the electron transfer within the complexes.

DOI: 10.1021/jp203576j

Ab Initio Study of Ternary Complexes X:(HCNH)+:Z with X, Z = NCH, CNH, FH, ClH, and FCl: Diminutive Cooperative Effects on Structures, Binding Energies, and Spin-Spin Coupling Constants Across Hydrogen Bonds

Ab initio calculations have been performed on a series of complexes in which (HCNH)⁺ is the proton donor and CNH, NCH, FH, ClH, and FCl (molecules X and Z) are the proton acceptors in binary complexes X:HCNH⁺ and HCNH⁺:Z, and ternary complexes X:HCNH⁺:Z. These complexes are stabilized by C–H⁺···A and N–H⁺···A hydrogen bonds, where A is the electron-pair donor atom of molecules X and Z. Binding energies of the ternary complexes are less than the sum of the binding energies of the corresponding binary complexes. In general, as the binding energy of the binary complex increases, the diminutive cooperative effect increases. The structures of these complexes, data from the AIM analyses, and coupling constants ¹J(N–H), ^1hJ(H–A), and ^2hJ(N–A) for the N–H⁺···A hydrogen bonds, and ¹J(C–H), ^1hJ(H–A), and^2hJ(C–A) for the C–H⁺···A hydrogen bonds provide convincing evidence of diminutive cooperative effects in these ternary complexes. In particular, the symmetric N···H⁺···N hydrogen bond in HCNH⁺:NCH looses proton-shared character in the ternary complexes X:HCNH⁺:NCH, while the proton-shared character of the C···H⁺···C hydrogen bond in HNC:HCNH⁺ decreases in the ternary complexes HNC:HCNH⁺:Z and eventually becomes a traditional hydrogen bond as the strength of the HCNH⁺···Z interaction increases.

DOI: 10.1021/jp2094164

DOI: 10.1021/jp202917z

DOI: 10.1016/j.cplett.2011.07.043

³¹P-³¹P spin-spin coupling constants for pnicogen homodimers

Ab initio calculations have been carried out in a systematic investigation of pnicogen homodimers (PH₂X)₂, for X = F, OH, NC, NH₂, CCH, CN, CH₃, H, and BH₂. Complex binding energies range from 7 to 34 kJ mol⁻¹, which is within the range observed for neutral hydrogen-bonded complexes. One-bond spin–spin coupling constants across the pnicogen interaction ^1pJ(P–P) exhibit a quadratic dependence on the P–P distance, similar to the dependence of ^2hJ(X–Y) on the X–Y distance for complexes with X–H⋯Y hydrogen bonds. Thus, computed values of ^1pJ(P–P) could be used to extract P–P distances from experimentally measured coupling constants.

DOI: 10.1080/00268976.2011.582050

Cooperativity between the hydrogen bonding and halogen bonding in F3CX...NCH(CNH)...NCH(CNH) complexes (X=Cl, Br)

MP2 calculations with the cc-pVTZ basis set were used to analyse the intermolecular interactions in F₃CX ··· NCH(CNH) ··· NCH(CNH) triads (X=Cl, Br), which are connected via hydrogen and halogen bonds. Molecular geometries, binding energies, and infrared spectra of the dyads and triads were investigated at the MP2/cc-pVTZ computational level. Particular attention was given to parameters such as the cooperative energies, cooperative dipole moments, and many-body interaction energies. All studied complexes, with the simultaneous presence of a halogen bond and a hydrogen bond, show cooperativity with energy values ranging between −1.32 and −2.88 kJ mol⁻¹. The electronic properties of the complexes were analysed using the Molecular Electrostatic Potential (MEP), electron density shift maps and the parameters derived from the Atoms in Molecules (AIM) methodology.

DOI: 10.1039/C1CP20560A

The boron-boron single bond in diborane(4) as a non-classical electron donor for hydrogen bonding

An ab initio study of an isomer of diborane(4) [B₂H₄] has been carried out at MP2/aug-cc-pVTZ to investigate the ground-state properties of this unusual molecule, a derivative of which has been described in the recent literature. The geometric, electronic and orbital characteristics of B₂H₄(4)have been analyzed using AIM, NBO, and ELF methodologies. A region with a high concentration of electron density is located near and along the B–B bond, on the opposite side of this bond relative to the bridging H atoms. This site serves as an electron-donor site to electrophiles, resulting in hydrogen-bonded complexes of B₂H₄ with proton donors HF, HNC,HCl, HCN, and HCCH, and a van der Waals complex with H₂. These complexes have C_2vsymmetry and stabilization energies that vary from 2 to 27 kJ mol⁻¹. The SAPT2 energydecomposition analysis shows that the relative importance of the various terms that contribute to the interaction energy depends on the strength of the interaction.