Energetic and Topological Analyses of Cooperative σH- and πH-Bonding Interactions

Abstract

A recent high-level ab initio study (J. Phys. Chem. A 2002, 106, 567) analyzed the formation of cooperative πH- and σH-bonding motifs in MP2/6-311++G(2d,2p)-optimized complexes of either one or two water molecules with ethene, propene, or allyl alcohol. The present study explores energetics and electron density redistributions associated with hydrogen bonding interactions in these clusters. Despite a substantial correlation component in the binding energy, the nonadditive three-body term, a descriptor of cooperative effects, is completely accounted for at the Hartree−Fock level. Natural bond orbital analysis attributes this cooperativity to the nonadditive character of charge-transfer interactions among local bond orbitals. Topological analysis of the electron density performed using Bader's theory of atoms in molecules confirms the closed shell, hydrogen-bonding nature of OH···π interactions and indicates their additional floppiness compared to conventional hydrogen bonds. The extension of these calculations to larger molecular systems validates the formation of a cooperative network of πH and σH bonds between the interfacial region of ceramide and water molecules. The OH···π bond serves to tether a water molecule to the trans double bond of the sphingolipid, extending the hydrophilicity of the interfacial region. This may contribute to the differences in biological activity between ceramide and its saturated counterpart, dihydroceramide.