W0149

Clathrate Hydrates. Structures and Guest Molecule Disorder. K.A. Udachin, C.I. Ratcliffe, J.A. Ripmeester, National Research Council, Ottawa, Canada.

Clathrate hydrates with low melting points (often below –20°C) are difficult subjects for single crystal data collection. A high level of guest molecule disorder inside the high symmetry cages causes difficulty for structure determination of such crystals as well.

Recent advances in single crystal X-ray diffraction have allowed this technique to be used as a valuable tool for the analysis of hydrate structure and composition. With detailed analysis of guest disorder, not only the guest positions are clearly defined, but also by using the cage occupancy as a free parameter it becomes possible to obtain estimates of hydrate composition from the absolute cage occupancies. In this way, it is found that the small cage in ethane hydrate is weakly occupied. The tilt angle between the ethane C-C bond and the equatorial plane of the large cage was found to be 23°, in good agreement with the value of 25.2° found previously from NMR studies. For the structure H hydrate of methylcyclohexane and methane, methane fills both small cavities to about 80% occupancy. For the cubic structure II hydrate of benzene and xenon, the orientation of benzene in the structure II large cage was located and xenon was confined strictly to the small cage. Tetrahydropyran in the large cage of the cubic structure II hydrate was shown to float in a boat conformation on the wall of the large cage.