13C solid-state MAS NMR studies of the low temperature phase transition in fullerene C60

Abstract

At the phase transition near 262 K, the 13C NMR spin–lattice relaxation time, T1, in static samples of pure C60 decreases from 31 to 0.8 s as the temperature decreases by less than 8 K, while X-ray diffraction patterns change little. Under magic-angle spinning T1 decreases from 46 to 0.8 s over only 2 K, while the onset of the phase transition is lowered to 255 K. NMR lineshapes of static samples are unchanged in the 280–235 K range, but the isotropic 13C chemical shift changes linearly by 1.88×10−3 ppm K−1. Uniaxial cage motion cannot average the chemical shift anisotropy to the value observed in static samples, and nutation must be present. Our tentative explanation of the results is based on differences in the nature of C60 cage motions at different temperatures.