Fourier transform NMR in liquids at high pressure. III. Spin–lattice relaxation in toluene-d8

Abstract

The spin–lattice relaxation time (T1) of both ring and methyl deuterons have been measured in toluene-d8 by the Fourier transform inversion recovery method. Measurements were made over the temperature range −35 to 200 °C, and over the pressure range 1 bar to 4 kbar. Viscosities and densities were also measured over most of this range. The T1 data are analyzed in terms of correlation times for isotropic overall molecular reorientation and internal rotation of the methyl group. Using pressure as a variable enables us to separate effects of density and temperature on overall and internal motion of the toluene molecule. Activation energies and activation volumes are reported. The experimental data are interpreted in terms of various theoretical models for the motional dynamics. The rotational diffusion model and the Enskog theory are used to discuss overall motions. For the case of internal motion of the methyl groups, the Langevin model is also considered. Large angle reorientation appears to be important in the overall motion of the toluene molecule at temperatures above 50 °C, whereas the motion of the methyl group is characterized by large angle steps even at lower temperatures. At high temperatures, the motion of the methyl group approaches that of a free rotor.