Divalent carbon atom as the proton acceptor in hydrogen bonding.

Proton-accepting properties of the divalent carbon atom in carbodiphosphoranes and their simple derivatives as well as in carbenes have been investigated. Both these groups of chemical compounds may be characterized by the formula CL2, where L is a sigma electron donor. Therefore, the carbon atom within both these systems, being in its atomic state, can have one or two lone electron pairs and, as a result, it may form hydrogen bonds of the type D-H...CL2, where C acts as a proton acceptor. Complexes of C(NH3)2, C(PH3)2, C[P(CH3)3]2, CF2, CCl2, and imidazol-2-ylidene with such proton donors as H2O, HCF3, HCN and HCCH have been analyzed by means of high-level quantum chemical methods. Density functional theory (DFT) and second-order Møller-Plesset (MP2) approaches have been applied in conjunction with the aug-cc-pVTZ basis set. The electron density distribution calculated by means of the atoms in the molecules procedure has also been analyzed. Proton-accepting properties of the carbon atom are discussed in detail. It is shown that the divalent carbon atom in the group of chemical systems investigated should be treated as a normal proton acceptor, similar to the much more electronegative O or N atoms. Moreover, hydrogen bonds of the type D-H...CL2 within the complexes investigated have been found to be rather strong. The highest proton accepting ability of the carbon(0) atom found for the (NH3)2C derivative of carbodiphosphorane is explained on the basis of the Leffler-Hammond postulate. Within the group of carbenes, the strongest hydrogen bonds are formed by imidazol-2-ylidene. This is attributed to the significant aromatic character of the imidazol-2-ylidene ring that increases the proton-accepting properties of the carbene carbon atom.