Motion of CO molecules in solid C-60 probed by solid-state NMR

Polycrystalline C-60 has been intercalated with 99 atom % C-13-enriched CO gas by using high-pressure, high-temperature synthesis. The ratio of (CO)-C -13 to C-60 is determined using C-13 NMR under magic-angle spinning and is found to be almost 1:2. Static solid-state C-13 NMR spectra of (CO)-C-13-in tercalated C-60 have been measured in the range between room temperature an d 4 K. In the high-temperature range, i.e., between room temperature and 10 0 K, the CO molecules in the "octahedral" sites of the C-60 lattice reorien t rapidly on the NMR time scale. At 4 K the reorientation rate of CO is so low that, on the NMR time scale, the molecule appears localized in one of t he minima of the potential. Between LC and 30 K, the transition from the st atic to the dynamic regime can be inferred from NMR line shape changes. The temperature dependence of the line shape is modeled in terms of thermally activated jump-like reorientations of the CO molecules in the C-60 lattice. No evidence for a quantum mechanical coherent tunneling motion of the CO m olecules in the octahedral sites of the C-60 lattice has been found.