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

Temperature dependence of F-19 spin-lattice and spin-spin relaxation times in hexafluoroethane, C2F6, was measured between 77 and 185K. 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.