Motion of CO Molecules in Solid C₆₀ Probed by Solid-State NMR

Polycrystalline C60 has been intercalated with 99 atom % 13C-enriched CO gas by using high-pressure, high-temperature synthesis. The ratio of 13CO to C60 is determined using 13C NMR under magic-angle spinning and is found to be almost 1:2. Static solid-state 13C NMR spectra of 13CO-intercalated C60 have been measured in the range between room temperature and 4 K. In the high-temperature range, i.e., between room temperature and 100 K, the CO molecules in the “octahedral” sites of the C60 lattice reorient 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 the minima of the potential. Between 4 and 30 K, the transition from the static 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 C60 lattice. No evidence for a quantum mechanical coherent tunneling motion of the CO molecules in the octahedral sites of the C60 lattice has been found