Dynamic Properties of the Tetrahydrofuran Clathrate Hydrate, Investigated by Solid State 2H NMR Spectroscopy

Abstract

Clathrate hydrates are solid inclusion compounds in which cages formed by a water host structure accommodate guest molecules of appropriate size and shape. Dynamic properties of the clathrate hydrate containing tetrahydrofuran (THF) guest molecules have been investigated using solid-state 2H NMR techniques, including line shape analysis and spin−lattice relaxation time (T1) measurements. 2H NMR results for THF−D2O suggest that there are at least two dynamically distinguishable types of water molecule, both undergoing four-site tetrahedral jump motions but with different rates. The population 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 motion regime (10-7 s ≲ τc ≲ 10-3 s). From analysis of the 2H NMR line shapes, the jump rates have been determined as a function of temperature, and activation parameters have been estimated. The 2H NMR spectrum for THF-d8−H2O comprises a single line, which broadens and changes shape on decreasing temperature. These observations are interpreted in terms of the THF-d8 guest molecules undergoing an approximately isotropic motion in the rapid regime (τc ≲ 10-7 s), but with a distribution of slightly anisotropic characteristics. As the guest molecules reorient rapidly in comparison with the dynamics of the water molecules, different guest molecules are effectively surrounded by structurally different water cages so that there is a distribution in terms of the anisotropic characteristics associated with the reorientational motions of different guest molecules. These conclusions are complemented by information determined from 2H NMR spin−lattice relaxation time measurements.