Simple carbenes as hydrogen bond acceptors: <i>ab initio</i> 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, <i>cyclo</i>-(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, <i>N</i>_B, of the carbenes were determined graphically from equilibrium dissociation energies, <i>D</i>_e, for the process B⋯HX = B + HX by using the equation <i>D</i>_e = <i>c</i>·<i>N</i>_B·<i>E</i>_HX, where <i>c</i> = 1.0 kJ mol^-1 and the <i>E</i>_HX are known numerical electrophilicities of the Lewis acids HX. <i>D</i>_e values were calculated <i>ab initio</i> 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 <i>N</i>R₂M 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, <i>N</i>_B/<i>σ</i>_axial, were determined by using the molecular electronic surface potential <i>σ</i>_axial at atom M (which lies on the <i>C</i>₂ 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 <i>cyclo</i>-(CH)₂-M carbenes, the determined values of <i>N</i>_B/<i>σ</i>_axial are shown to be independent of R and M.