Simple carbenes as hydrogen bond acceptors: 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 = RM, -(CH)M, HCCM and two N-heterocyclic carbenes, where M is one of the group 14 tetrel atoms, C, Si, Ge or Sn and R = H, CH, and F. All the carbenes but CH have a singlet electronic ground state. The Lewis acids, HX, involved are HF, HCl, HBr, HI and HCN, HCCH, and HCP. Nucleophilicities, , of the carbenes were determined graphically from equilibrium dissociation energies, , for the process B⋯HX = B + HX by using the equation = ··, where = 1.0 kJ mol and the are known numerical electrophilicities of the Lewis acids HX. values were calculated at the CCSD(T)-F12c/cc-pVDZ-F12 level of theory, which for CH refers to the singlet electronic excited state. It was established that RM values lie in the order of M = C ≫ Si ∼ Ge ∼ Sn for a given R and in the order R = CH > H > F for a given M. Reduced nucleophilicities, /, were determined by using the molecular electronic surface potential at atom M (which lies on the axis) on the 0.001 e Bohr iso-surface of each carbene molecule, as calculated at the MP2/aug-cc-pVTZ level. For RM having R = CH and H and -(CH)-M carbenes, the determined values of / are shown to be independent of R and M.