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.