Effects of Temperature on Cs+(H2O)20 Clathrate Structure

Clusters consisting of 20 water molecules and a single cesium ion are especially stable due to their clathrate structure that is composed exclusively of three-coordinate water molecules. Clathrate stability was investigated using infrared photo-dissociation (IRPD) spectroscopy in the free-OH stretching region (similar to 3600-3800 cm(-1)) at ion cell temperatures between 135 and 355 K. At 275 K and colder, IRPD spectra of Cs+(H2O)(20) have just one acceptor-acceptor-donor band. At higher temperatures, a higher-energy acceptor-donor band emerges and grows in intensity. Non-clathrate Na+(H2O)(20) structures contain both of these bands, which do not change significantly in intensity over the temperature range. These results indicate a rapid onset in the conversion from clathrate to non-clathrate structures with temperature and suggest that some clathrate population remains even at the highest temperatures investigated. These results provide new insights into the role of entropy in clathrate stability.