Expansion and Shrinkage of Water Cages in Structure

Expansion and Shrinkage of Water Cages in Structure II Clathrate Hydrates: The Effects of Temperature and Guest Camille Y. Jones, Hamilton College, Clinton, NY 13323 In our current work on the synthesis of structure II hydrates for neutron diffraction studies, we observe that the conditions for the stability of hydrogenated and deuterated hydrates display marked differences, not related to the difference between the melting points of H 2 O and 2 H 2 O in a simple way. This brought us to an important question: what differences might the crystal structures themselves display? This is an important issue in light of the fact that structures derived from neutron scattering data involve the use of 2 H 2 O. To consider this question in some detail, we have performed a detailed study of the water host of structure II clathrate hydrate with PO as the guest (Fig. 1) recently determined from high-resolution, constant-wavelength neutron powder diffraction data. Here we found the typical structure II framework but with a range of bond distances and angles (Fig. 2) that resembles high-pressure ices and ionic salts more than those of ice Ih. At low temperatures, where guest motion is negligible, the bond distances for the filled cages are most similar to those observed for 2 H 2 O ice Ih. The empty cages appear to be more highly distorted. Although these temperature-dependent distortions are no doubt related to the size, shape, and dynamics of the guest molecules as well as the dynamics of the host itself, they might also be related to a 2 H-isotope effect on the water dynamics. In addition, variations in the bond angles and distances may provide a quantitative indicator of the strength of guest-host interactions or the deviation of thermodynamic properties of hydrate hosts from that of ice Ih; here again, isotope effects will have to be taken into consideration. Further elucidation will require the analysis of static vs. dynamic disorder (in progress) and determination of the structure of hydrogenated PO hydrate with X-rays. Figure 1. Fragment of propylene oxide (PO) clathrate hydrate showing PO guests trapped in large hexakaidecahedral cages. The smaller dodecahedral cage is empty. (C. Y. Jones, J. Curtis, and I. Peral, unpublished results) Figure 2. Bond distances and angles for the large (HEX) and small (DOH) cages in structure II PO hydrate. The blue shaded areas identify the range of values in ice Ih, orange for the range of values in ionic hydrates. Each symbol represents a crystallographically unique distance or angle in the structure