COMPUTER-SIMULATIONS OF CESIUM WATER CLUSTERS - DO ION WATER CLUSTERSFORM GAS-PHASE CLATHRATES

The structure and energetics of cesium ion-water clusters have been in vestigated using classical molecular dynamics computer simulations and a polarizable interaction model. Recent experiments by Selinger and C astleman [J. Phys. Chem. 95, 8442 (1991)] indicate that the mass-spect ral distributions for these clusters exhibit ''magic number'' oscillat ions at temperatures below approximately 160 K. The observed behavior of this and related charged clusters is commonly attributed to the for mation 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 temperatur e for clusters with between 18 and 22 water molecules. The clusters ex hibit solidlike dynamical behavior at kinetic temperatures below about 170 K, and liquidlike behavior at higher temperatures. A thorough ana lysis of energy minimized (0 K) structures indicates that the most sta ble 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 w ith cluster size that is consistent with the experimentally observed m ass-spectral distributions. In contrast, in the liquidlike regime ther e are only minor variations in calculated structural and dynamical pro perties as a function of cluster size. In addition, there is no statis tically significant size dependence for the cluster binding energies i n the liquidlike regime that might be correlated with experimental dat a. These results suggest that magic number stability in ion-water clus ters may occur only at ''low'' energies in the solidlike cluster regim e.