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Nuclear magnetic resonance studies on clathrate hydrates Raghunathan, Parthasarathy

Abstract

With a view to obtaining information on the nature and extent of molecular motion of enclathrated "guests" and their interaction with the "host" lattices, nuclear magnetic resonance absorption of ten guest species included in the clathration voids of fully deuterated hydrates has been studied from a temperature of 77ºK upwards. The F¹⁹ resonance line shapes for CF₄ and SF₆ and the H¹ resonance line shape for ethylene oxide (C₂H₄0) in their respective clathrate hydrates indicate that these molecules are but little restricted by the walls of the clathrate cavities, and reorient freely about chosen axes of symmetry at low temperatures and at random at higher temperatures. Above 150ºK, a limited isotropic translation, or "rattling", of an SF₆ guest molecule up to a distance of [formula omitted] from the centre of the clathration volume has been demonstrated. Proton resonance has been studied for propane in two specimens of the clathrate hydrate, one of which was richer in guest content than the other. For these two specimens it has been suggested that, below 160ºK, propane assumes a staggered C₂ configuration inside the clathrate cavity. Molecular C₂ - axis reorientations superposed on methyl reorientations have been proposed, and a thermal activation energy barrier of 1.70 ± 0.08 kcal/ mole has been calculated for the above motion from spin-lattice relaxation time measurements in the 77ºK - 110°K range. Closer to the melting point of the two specimens, diffusion of propane through the host lattice has been indicated, and diffusional activation energies of 1.40 ± 0.02 kcal/mole and 0.75 ± 0.05 kcal/mole have been obtained for the guest - rich and guest - poor specimens, respectively. In sharp contrast to the above results, the low temperature proton resonance of three halomethanes, CH₃X(X = CI, Br, I), inside hydrate host cavities has revealed definite constraints to reorientational and translational motion, the second moment data indicating only low-amplitude oscillatory motions of the CH₃ groups in these clathrates. A triplet line shape has been observed for the CH₃Br clathrate at 77°K. At higher temperatures, expansion of the hydrate lattices has been proposed, which permits free C₃- reorientations of the CH₃ groups of the three guest molecules. From the associated linewidth transitions, activation energies of 2.48 ± 0.32, 9.30 ± 0.25, and 6.80 ± 0.50 kcal/mole have been calculated for the potential barrier hindering methyl reorientation in the CH₃Cl-, CH₃Br-, and [formula omitted] hydrates, respectively. The motional model proposed for this temperature range is adequately supported by measurements of H¹ spin - lattice relaxation times. For the dichloromethane clathrate hydrate, a clearly resolved doublet characteristic of rigid proton pairs has been obtained at 77°K. The possible existence at low temperatures of an aligned guest molecule in a suitably-sized cavity is thereby indicated. A line shape analysis of this doublet, performed on an IBM 7040 computer, yielded an accurate H-H interatomic distance of 1.73Å for the 'guest' dichloromethane molecule. This value has been discussed in the light of results from earlier microwave studies of dichloromethane. The proton resonance linewidth and second moment results between 77°K and 286°K for i-amyl groups included as guests in the clathrate hydrate of (i-C₅H₁₁)₄NF have been interpreted in terms of simple motional models of these guest moieties. The results complement the reported crystal structure of this clathrate. For the analogous hydrate containing 'guest' n-butyl groups, proton second moments in the same temperature range have supported the disordered guest structure reported from a previous x-ray diffraction study. In addition, hysteresis has been demonstrated in the second moment curve of this clathrate beyond 248°K, and this has been ascribed to a phase transition.

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