Non-additivity of OH frequency shifts in ion-water systems

Abstract

The intramolecular OH stretching frequency shift observed for water on formation of ‘cation-water…H-bond acceptor’ complexes is highly non-additive. The uncoupled OH frequency shift has been calculated by ab initio methods for q +-0H 2 and OH 2… q − complexes of different geometries and for a series of Mg 2+·(H 2O) n …(H 2O) m complexes, and we argue that the non-additivity (cooperativity) of the frequency shifts is largely a consequence of the parabolic shape of the frequency versus electric field curves.