Dynamic properties of the tetrahydrofuran clathrate hydrate, investigated by solid state H-2 NMR spectroscopy

Clathrate hydrates are solid inclusion compounds in which cages formed by a water host structure accommodate guest molecules of appropriate size and s hape. Dynamic properties of the clathrate hydrate containing tetrahydrofura n (THF) guest molecules have been investigated using solid-state H-2 NMR te chniques, including line shape analysis and spin-lattice relaxation time (T -1) measurements. H-2 NMR results for THF-D2O suggest that there are at lea st two dynamically distinguishable types of water molecule, both undergoing four-site tetrahedral jump motions but with different rates. The populatio n ratio may be assigned as 3:1, consistent with the known crystal structure . Above 193 K, the dynamics of both components are in the intermediate moti on regime (10(-7) s less than or similar to tau (c) less than or similar to 10(-3) s). From analysis of the H-2 NMR line shapes, the jump rates have b een determined as a function of temperature, and activation parameters have been estimated. The H-2 NMR spectrum for THF-d(8)-H2O comprises a single l ine, which broadens and changes shape on decreasing temperature. These obse rvations are interpreted in terms of the THF-d(8) guest molecules undergoin g an approximately isotropic motion in the rapid regime (tau (c) less than or similar to 10(-7) s), but with a distribution of slightly anisotropic ch aracteristics. As the guest molecules reorient rapidly in comparison with t he dynamics of the water molecules, different guest molecules are effective ly surrounded by structurally different water cages so that there is a dist ribution in terms of the anisotropic characteristics associated with the re orientational motions of different guest molecules. These conclusions are c omplemented by information determined from H-2 NMR spin-lattice relaxation time measurements.