College Park, Maryland      June 6 - 10 , 2004

M2-D4 (11:45): Structural Distortions and Guest Dynamics of Clathrate Hydrates

C. Y. Jones (NIST Center for Neutron Research), I. Peral (NIST Center for Neutron Research; Department of Materials Science and Engineering, University of Maryland)

Gas hydrates are water-based, natural gas-containing inclusion compounds that occur in nature in vast deposits under the ocean floor and in the arctic permafrost.(1) They belong to a larger family of inclusion compounds known as clathrate hydrates. The structure of a clathrate hydrate (CH) comprises water molecules held together by hydrogen bonds in a three-dimensional "host" network of spherical polyhedra, or cages, which contain a small "guest" molecules. Neutron powder diffraction studies of a CH (C4H8O·17D2O) with the guest tetrahydrofuran (THF) in the range (7-165) K have shown that distortions occur in the host as a function of temperature (T).(2) In particular, distortions at low T brings D atoms closer than the O atoms to the center of the cage, and indicate that changes in the specific configuration of O and D may be important in the determination of the guest-host interactions and the resulting CH properties. An explanation for the distortions is suggested in the findings from quasielastic neutron scattering studies (QENS) of trimethylene oxide (TMO) CH, chemical formula C3H6O·17D2O.(3) The QENS results show distinct regions of high-T and low-T rotational dynamics, and an activation energy for guest rotation which is higher in the high-T regime than at low T. This presentation describes recent neutron diffraction and QENS results in the range (5-260) K on the CHs of the cyclic ethers THF, TMO, and propylene oxide. A relationship between the host structural distortions and the changes in the guest dynamics is proposed.

1. Makagon, Y. F.; Trebin, F. A.; Trofimuk, A. A.; Tsarev, V. P.; Chersky, N. V.; Dokl. Acad. Sci. USSR -- Earch Sci. Sect. 1972, 196, 197-200.
2. Jones, C. Y.; Marshall, S. L.; Chakoumakos, B. C.; Rawn, C. J.; Ishii, Y. “Structure and Thermal Expansivity of Tetrahydrofuran
     Deuterate Determined by Neutron Powder Diffraction.” J. Phys. Chem. B 2003, 107(25), 6026-6031.
3. Jones, C. Y.; Peral, I. “Dynamics of Trimethylene Oxide in a Structure II Clathrate Hydrate.” Submitted to The American Mineralogist.

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