International Conference on Gas Hydrates (ICGH) (6th : 2008)

AB INITIO STRUCTURE DETERMINATION OF GAS HYDRATES AND REFINEMENT OF GUEST MOLECULE POSITIONS BY POWDER X-RAY DIFFRACTION Takeya, Satoshi; Udachin, Konstantin A.; Ripmeester, John A.

Abstract

Structure determination of powdered crystals is still not a trivial task. For gas hydrates, the difficulty lies in how to determine the rotational disorder and cage occupancies of the guest molecules without other supporting information or constraints because the complexity of the problem for the powder diffraction technique generally depends on the number of atoms to be located in the asymmetric unit. Here, the crystal structures of gas hydrates of CO2, C2H6, C3H8, and Methylcyclohexane/CH4, as determined by the direct-space and Rietveld techniques are reported. The resultant structures and cage occupancies were consistent with results found from conventional experimental methods using single crystal x-ray diffraction or solid-state 13C-NMR. It was shown that the procedures reported in this study make it possible to determine guest disorder and absolute cage occupancy of gas hydrates even from powder crystal.

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