Structure and thermal expansivity of tetrahydrofuran deuterate determined by neutron powder diffraction

The crystal structure of tetrahydrofuran deuterate, a clathrate hydrate, has been refined from neutron powder diffraction data at five temperatures in the range 7-265 K. The thermal expansivity was shown to be greater than that of ice Ih in the same range of temperature (7), as observed in previous studies of other clathrates. The overall effect of T has been resolved into contributions from different geometrical parameters in the structure. Thus, an increase in T results in expansion of the host-lattice framework with increases in both the D-D and O-O distances and out-of-plane tilting of water molecules. The greatest dependence on T is exhibited by the D-D distances and the distortion of the hexagonal faces from planarity, which is particularly pronounced in the range 75-140 K. The cage volumes show a complex dependence on T. from 7 to 140 K, the volume of the small cage decreases slightly and that of the large cage increases, and between 140 and 205 K, the trend is reversed. The most pronounced structural changes occur in a similar regime of T as changes in guest dynamics observed in spectroscopic and thermodynamic studies. The temperature dependences of the structure and alpha(T), when considered along with the relation of (x(T) to the degree of anharmonicity in bonding,(34) could be formulated to provide a sensitive test of molecular models of clathrate hydrates.