Abstract
Evidence is presented which suggests that the thermally activated reorientation rate tau -1 of methyl groups in a very wide range of solids closely follows a universal law which relates tau -1 to the hindering barrier height V and the temperature theta . Thus tau -1 simply measures theta with a calibration curve dependent on V. A simple theory of the calibration curves is described, using a thermal average over all methyl group torsion-rotation states of the modulus of the expectation value of angular momentum. No adjustable parameters are involved if a simple cosine hindering potential is assumed. The relationship is tested on a large number of methyl-containing solids for which V has been deduced from measurements of the tunnel splitting at low temperatures. The predicted temperature-dependent reorientation rate turns out to be very similar to that given by a modified version of the theory of Stejskal and Gutowsky (1958). The relationship between the theories is discussed. Comparison is made with experimental data derived from magnetic resonance relaxation time measurements, and from inelastic neutron scattering studies.