ANISOTROPIC MOLECULAR-REORIENTATION OF LIQUID BENZENE REVISITED - A STUDY USING C-13 MAGNETIC-RELAXATION THROUGH CHEMICAL-SHIFT ANISOTROPY AND SPIN ROTATION

We report on C-13 magnetic relaxation in liquid benzene (C6H6 and C6D6) at 293 K at resonance frequencies of 22.63 and 100.6 MHz. The total spin-lattice relaxation rates are separated into contributions from C-13-H-1 and C-13-2D dipolar, spin-rotation (SR), and chemical shift anisotropy (CSA) interactions. As the principal axis of the C-13 chemical shift tensor is collinear with the sixfold axis of benzene, CSA relaxation provides information on the tumbling motion of benzene, hitherto not available from other NMR experiments. The resulting correlation time tau-perpendicular-to = 1.63 ps at 20-degrees-C indicates a faster tumbling motion, and hence, a smaller anisotropy of benzene reorientation than usually quoted in the literature. The results is qualitatively confirmed by an analysis of C-13 relaxation by SR interaction, and is in fair agreement with recent results obtained in Raman line shape measurements. It is, however, at variance with the figures deduced from Rayleigh scattering experiments. The activation energy for the tumbling motion is found to be 9.8 kJ mol-1, which is markedly larger than the activation energy of 7.7 kJ mol-1 obtained from 2D relaxation data for the motion of in-plane axes.