Molecular Motion and Plastic-phase Transition of Crystalline C2F6 as Studied by 19FNMR

Abstract

Temperature dependence of ¹⁹F spin-lattice and spin-spin relaxation times in hexafluoroethane, C₂F₆, was measured between 77 and 185 K. An abrupt change in T ₁ was observed at the transition temperature (103.98 K). Reorientation of CF₃ groups takes place with the activation energy (E_a ) of 21.6 kJ mol⁻¹ in the low temperature phase. In the high-temperature plastic phase the overall rotation of the whole molecule occurs with E_a = 2.0 kJ mol⁻¹, being followed by the self-diffusion of the molecule (E_a = 30.0 kJ mol⁻¹). It is noted that the low-temperature phase transforms to the plastic phase before the averaging out of the anisotropic intermolecular interactions occurs in the C₂F₆ crystals.