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

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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 (Ea ) 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 Ea = 2.0 kJ mol−1, being followed by the self-diffusion of the molecule (Ea = 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.