Intermolecular charge transfer as evidence for unusual O–H⋯C(sp3) hydrogen bond

Abstract

Quantum chemical calculations of the HOH⋯CH4 complex, in which water acts as the proton donor and methane is the acceptor, were performed at different levels of theory. The NBO analysis reveals the interaction between σ orbitals of the covalent C–H bonds of methane and σ∗ antibonding orbital of the O–H bond of water associated with the charge transfer from methane to water. The values of topological parameters of the bond critical point for the H⋯C contact are typical for neutral H-bonded molecular complexes. The binding energy in the complex calculated at the MP2/aug-cc-pVQZ level with the BSSE correction was found to be 0.93kcal/mol. With protonated water, the binding energy is raised up to 10kcal/mol. In the H2OH+⋯CH4 complex of hydronium with methane, the O–H⋯σ(C–H) interaction shows features similar to those of hydrogen bonds in the complexes of a hydronium ion with HCl and H2S molecules.