NMR Studies of Guest Dynamics in Clathrate Hydrates: Spherical Tops SF6, SeF6 and CH4 in Structure II Hydrate

Spin−lattice relaxation time measurements, obtained between 2 and ∼150 K, are presented for 19 F in two structure II clathrate hydrates, SF 6 ·17D 2 O and SeF 6 ·17D 2 O, as well as for 1 H in a double structure II deuteriohydrate of methane and tetrahydrofuran- d 8 . The 19 F 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 19 F 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 1 H relaxation times, measured for CH 4 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.