Molecular dynamics of C-60.2S(8): A C-13 NMR study

C-13 NMR studies of polycrystalline C-60. 2S(8) are reported. At room tempe rature, the static C-13 NMR spectra are similar to the C-13 spectrum of pol ycrystalline C-60, which suggests that C-60 molecules undergo fast reorient ational motion within the solvate lattice as within pure crystalline C-60 T his observation seems to contradict previous x-ray results that were interp reted in terms of motionless C-60 molecules within the solvate lattice; how ever, it agrees with thermodynamic measurements according to which the enth alpy for the departure of sulfur from the solvate is nearly equal to that o f pure sulfur sublimation. The evolution of the shielding tensor as a funct ion of temperature is also reported: it exhibits a steep change near 150 K that corresponds to a slowing down of the C-60 rotational motion. Finally, according to the variation of the C-13 spin-lattice relaxation time T-1 vs temperature, the dynamics of C-60 molecules, which consists in jumps betwee n symmetry-equivalent orientations, can be considered as ruled by two activ ation energies, 15.3 and 24.8 kJ mol(-1).