Single Crystal Diffraction Studies of Structure I, II and H Hydrates: Structure, Cage Occupancy and Composition

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 are the guest positions clearly defined, but 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 (∼5%), something not seen in previous NMR work. 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 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 structure II hydrate was shown to float in a boat conformation on the wall of the large cage. We also report lattice parameters for a number of structure II hydrates determined by single crystal diffraction. A rough trend of unit cell parameter with guest size or volume is illustrated, although it is not as yet clear if there are other correlating factors.