Molecular dynamics and the phase transition in solid ${\mathrm{C}}_{60}$

We characterize the molecular reorientational dynamics in two phases of solid ${\mathrm{C}}_{60}$ with ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR measurements. A change in the nature of the dynamics, indicated by a change in kinetic parameters extracted from spin-lattice relaxation data, occurs at the phase transition at 260 K. Above the transition, the molecules appear to execute continuous rotational diffusion; below the transition, they appear to jump between symmetry-equivalent orientations. This interpretation is consistent with the x-ray-diffraction results of Heiney et al. as well as our NMR relaxation and spectral data.