| Résumé | Spin-lattice relaxation time measurements, obtained between 2 and ~150 K, are presented for 19F in two structure II clathrate hydrates, SF6-17D2O and SeF6-17D2O, as well as for 1H in a double structure II deuteriohydrate of methane and tetrahydrofuran-d8. The 19F results were analyzed in terms of three relaxation mechanisms related to the guest motions, including fluctuations in nuclear dipolar couplings and the anisotropic chemical shift, plus a contribution from spin rotation interactions above ~50 K. For quantitative agreement, the incorporation of a distribution function into the relaxation equations is required to account for the proton disorder in the hydrate cages, in this case a normal distribution in activation energies. The dynamic parameters derived from relaxation measurements also account for the motional narrowing of the 19F spectra. This is the first instance where a single motional model has accounted for both spectral narrowing and relaxation times for guest dynamics in clathrate hydrates over a wide range of temperatures. The 1H relaxation times, measured for CH4 in the small cages of structure II, are generally consistent with the above model, except that, because of quantum symmetry effects, only intermolecular dipolar couplings appear to be effective in providing relaxation. Trends in activation energies for guest reorientation and the associated distributions, as well as previously published work, are discussed in terms of our understanding of guest dynamics in clathrate hydrates. |
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