Structures, energies, and bonding in the microsolvation of Na+

Abstract

A stochastic search of the potential energy surfaces for the interctions between the sodium cation and up to six explicit water molecules was performed under Møller–Plesset second–order perturbation theory (MP2) with extended basis sets. At these low molecularties, the sodium cation is more likely to occupy the interior of the cluster, energetically favoring a maximum coordination number of four, even when five or six water molecules are available. An energy decomposition analysis reveals a major contribution from electrostatic interactions to cluster stabilization, with non-negligigle induction contributions accounting for ≈20% of the total interaction. Topological analysis of the electron distribution unambiguously describe H2O⋯Na+ ionic interactions. Hydrogen bond networks seem to arrange in order to maximize the stability of Na+ via a clear charge transfer pattern, where electron density is withdrawn from the O–H bonds towards Na+. Because of this reorganization of the electron density, and because of the perturbative effect of the formal charge, all H2O⋯H2O hydrogen bonds are also highly ionic in nature with some degree of covalency.