Computational study of proper and improper hydrogen bonding in methanol complexes

Abstract

The bulk properties of alcohols, like those of aqueous solutions, are governed mostly by hydrogen bonding; however, in contrast with water molecules, the chemical structure of a simple alcohol such as methanol offers an opportunity to explore the effects of both proper and improper hydrogen bonding on a single hydrogen donor. The presence of the hydroxyl group generally gives rise to a strong proper hydrogen bond, while the methyl group of methanol is likely involved in the weaker improper hydrogen bond, among other weak non-covalent interactions. The effects of the two types of hydrogen bonds on the stability, geometric parameters, and properties of electron density of methanol complexes are examined while considering different geometrical arrangements of the methanol dimer and the binary complexes of methanol with water, acetonitrile, and chloromethane. Subsequently, potential conclusions about the nature of improper hydrogen bonding and the origin of the C–H bond contraction that results upon complex formation are discussed. Quantum theory of atoms in molecules and natural bond orbital methods were used in the analysis; all calculations were performed at the MP2(full)/6-311++G(d,p) level of theory.