C-H⋯O, O-H⋯C, and C-H⋯C interactions in complexes of carbocations and carboanions

Molecular complexes formed by different forms of carbocations (carbenium ions) and carboanions with water, acetylene, and methane molecules have been calculated by the MP2/6-311++G(2df,2pd) method. In complexes with water where the carbon atom of the carbocation (carboanion) acts as the proton donor (acceptor), the energies of the C-H⋯O and O-H⋯C hydrogen bonds turn out to be approximately the same being 13–20 kcal/mol for carbocation (carboanion) species differing in the valence state of the carbon atom. Two types of C-H⋯C interactions have been revealed depending on the charge at the bridging hydrogen atom, which is determined by the hybridization of the donor carbon atom. The C-H⋯C interaction energy in molecular complexes with the positively charged hydrogen atom (carboanion complexes with acetylene) is an order of magnitude higher than in the complexes where the bridging hydrogen atom has an excess of electron density (carbocation complexes with methane). In all the complexes under consideration, the covalent C-H bond involved in interaction is elongated, and the negative charge is transferred from the acceptor to the donor.