A wide-line NMR study of reorientation of some spherical-top molecules enclathrated in water

Abstract

NMR spectra at temperatures down to 1.8 K are given for the ¹⁹F resonance of SF₆ and CF₄ in H₂O and D₂O clathrates and for the ¹H resonance in CH₄–D₂O and CH₄–tetrahydrofuran‐d₈–D₂O clathrates. The second moments correspond effectively to isotropic reorientation of encaged SF₆, CF₄, and CH₄ molecules at temperatures above 13, 22, and 4 K, respectively. The CH₄ spectra are only slightly broadened at 1.8 K. For SF₆ and CF₄ a low‐temperature transition in second moment is characterized by the superposition on the rigid lattice band of a narrow component whose intensity increases progressively with rise of temperature. This ’’apparent phase‐change effect,’’ after Resing, is attributed to a very broad distribution of reorientational correlation times, here associated with orientational disorder of the water molecules of the lattice. The behavior during the transition agrees with a model which assumes a Gaussian distribution of activation energies about a mean value of 207 cal/mol for SF₆ and 360 cal/mol for CF₄.