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

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, H₂C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, 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, N_B, of the carbenes were determined graphically from equilibrium dissociation energies, D_e, for the process B⋯HX = B + HX by using the equation D_e = c·N_B·E_HX, where c = 1.0 kJ mol^-1 and the E_HX are known numerical electrophilicities of the Lewis acids HX. D_e 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, N_B/σ_axial, were determined by using the molecular electronic surface potential σ_axial 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 N_B/σ_axial are shown to be independent of R and M.