Computer simulations of cesium–water clusters: Do ion–water clusters form gas-phase clathrates?

Abstract

The structure and energetics of cesium ion–water clusters have been investigated using classical molecular dynamics computer simulations and a polarizable interaction model. Recent experiments by Selinger and Castleman [J. Phys. Chem. 95, 8442 (1991)] indicate that the mass-spectral distributions for these clusters exhibit ‘‘magic number’’ oscillations at temperatures below approximately 160 K. The observed behavior of this and related charged clusters is commonly attributed to the formation of clathratelike cage structures around a central ionic species. The relationship between the structural and energetic properties of cesium ion–water clusters is reported here as a function of temperature for clusters with between 18 and 22 water molecules. The clusters exhibit solidlike dynamical behavior at kinetic temperatures below about 170 K, and liquidlike behavior at higher temperatures. A thorough analysis of energy minimized (0 K) structures indicates that the most stable clusters consist of water cages surrounding the cesium ion. These cages are related to the proposed clathratelike structures but contain additional 4- and 6-membered water rings and fewer 5-membered rings. The calculated global energy minima exhibit an energetic alternation with cluster size that is consistent with the experimentally observed mass-spectral distributions. In contrast, in the liquidlike regime there are only minor variations in calculated structural and dynamical properties as a function of cluster size. In addition, there is no statistically significant size dependence for the cluster binding energies in the liquidlike regime that might be correlated with experimental data. These results suggest that magic number stability in ion–water clusters may occur only at ‘‘low’’ energies in the solidlike cluster regime.