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Abstract

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.