NUCLEAR-RELAXATION AND NEUTRON-SCATTERING STUDY OF THE ROTATIONAL STATES OF METHYL-GROUPS IN SOLIDS WITH LOW HINDERING BARRIERS

The proton spin-lattice relaxation rate of toluene, of toluene diluted in toluene-d8 and of toluene-d5 diluted in toluene-d8 has been measured over the temperature range 10-75 K. The inelastic neutron scattering peak associated with torsional energy splittings of the methyl group has been studied at low temperature. Data of the random jump rotation has been obtained from the width of the quasi-elastic scattering peak between 30 and 80 K. The results confirm the basic ideas of the theoretical relaxation model of Haupt. By removing dipole interactions of methyl protons with protons outside the CH3 group the field dependent peak in the relaxation disappears. There is excellent agreement between NMR and neutron scattering in explaining the two types of activation process: random jump reorientation (1.3 kJ/mol) over the barrier and torsional excitation (0.55 kJ/mol) connected with tunnelling. Rotational potentials and energy levels have been derived from the experimental quantities using tabulated eigenvalues. © 1979.