Proton magnetic resonance absorption of halomethane “guest” molecules in clathrate hydrates

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, the nuclear magnetic resonance absorption of four halomethanes included in the clathration voids of fully-deuterated hydrates has been studied from 77°K. upwards. For the three monohalomethanes (CH 3X; X = Cl, Br, I) the low-temperature proton resonance results indicate constraints to reorientational and translational motion inside hydrate host cavities. A triplet line-shape has been observed for the CH 3Br-clathrate at 77°K. At higher temperatures, expansion of the deuterium-bonded hydrate lattices has been proposed, which permits free C 3-reorientations of the CH 3 groups of the 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 −1 have been calculated for the potential barrier hindering methyl reorientation in the CH 3Cl-, CH 3Br- and CH 3I-clathrates, respectively. The motional model proposed is supported by measurements of the 1H spin-lattice relaxation times.