MOLECULAR ORIENTATIONAL DYNAMICS IN SOLID C-70 - INVESTIGATION BY ONE-DIMENSIONAL AND 2-DIMENSIONAL MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE

We present the results of C-13 nuclear magnetic resonance (NMR) measur ements that probe molecular orientational dynamics in solid C70 in the temperature,range 223-343 K. Orientational dynamics affect the NMR li ne shapes and spin-lattice relaxation rates by modulating the C-13 che mical shift anisotropy (CSA). Motionally averaged CSA line shapes, det ermined from both one-dimensional and two-dimensional magic angle spin ning NMR spectra, and relaxation rates are determined for each of the five inequivalent carbon sites in the C70 molecule. Comparisons of the results for the five sites provide evidence for rapid uniaxial molecu lar reorientation in the monoclinic (T<280 K) and rhombohedral (280 le ss-than-or-equal-to T less-than-or-equal-to < 330 K) phases and rapid isotropic reorientation in the face-centered cubic (T greater-than-or- equal-to 330 K) phase. The orientational correlation time is roughly 2 ns at 250 K and of the order of 5 ps at 340 K.