Simple Carbenes as Hydrogen-Bond Acceptors: Nucleophilicities and Reduced Nucleophilicities determined ab initio

Nucleophilicities for a range of simple carbenes molecules acting as hydrogen-bond acceptors B in forming complexes BHX are reported. The carbenes chosen to fulfil the roles of 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, 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, 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, that for CH2 referring to the singlet electronic excited state. It was established that NR2M values lie in the order 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 the atom M (which lies on the C2 axis) on the 0.001 e/bohr3 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.