College Park, Maryland      June 6 - 10 , 2004

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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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