WATER-CLUSTER DISTRIBUTION WITH RESPECT TO PRESSURE AND TEMPERATURE IN THE GAS-PHASE

In spite of decades of extensive studies of water, the experimental in formation of water clusters larger than the trimer is hardly available yet. To aid the better analysis of certain cluster properties in an e xperiment, we studied the small-water-cluster distribution (particular ly for the minimum-energy structures) in the gas phase. Utilizing the thermodynamic information in the range from the water monomer to the o ctamer (except for the heptamer) by ab initio calculations, we investi gated the mole fractions of the water clusters along the vapor pressur e of the condensed phase. These mole fractions increase with increasin g temperature or pressure, while the higher clusters increase still mo re. The entropy increment of the cyclic pentamer relative to the cycli c tetramer is particularly small; thus, the cyclic pentamer shows ther modynamically unusual characteristics. For the trimer, the cyclic stru cture is more stable than the linear structure at temperatures lower t han approximately 400 K, while above this temperature, the latter is m ore stable due to the entropy effect. Similar phenomena are also expec ted for the higher clusters. The mole fractions of the higher cyclic c lusters are found to be very small in a vapor unless they are unconden sed with insufficient water molecules.