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

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+-0H2 and OH2…q− complexes of different geometries and for a series of Mg2+·(H2O)n…(H2O)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.