NUCLEAR MAGNETIC-RESONANCE STUDIES OF GUEST SPECIES IN CLATHRATE HYDRATES - LINE-SHAPE ANISOTROPIES, CHEMICAL-SHIFTS, AND THE DETERMINATION OF CAGE OCCUPANCY RATIOS AND HYDRATION NUMBERS

NMR spectra of the guest molecules PH3, H2Se, D2Se, D2S, CD3F, CD3Cl, CD3Br, C2D2, and C2D4 in their structure I clathrate hydrates have been obtained by use of 2H, 19F, 31P, and 77Se nuclei. Components due to guests in the small and large cages have been distinguished by using isotropic chemical shift and static line-shape anisotropy differences. Low-temperature magic angle spinning was used in some cases to resolve the two components. Guests in the small cages are invariably found to have a lower field isotropic shift than those in the large cage. The static line shapes are isotropic for guests in the small spherical cages, whereas in the large oblate cages they have a residual anisotropy. Relative cage occupancy ratios θS/θL have been obtained from the observed NMR intensities, and together with similar results from previous NMR studies, these have been used to derive hydration numbers. This represents a new and nondestructive method of determining structure I hydrate compositions.