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.