The correlation of methyl tunnelling and thermally activated reorientation

Abstract

A strong correlation is demonstrated between two parameters connected with methyl group motion, and measured in a wide variety of solids. The parameters are h nu _t, the tunnel splitting of the ground torsional state, and theta _min, the temperature at which the rate constant tau ^-1 associated with thermally activated reorientation has the value 2*10⁸s^-1. The results suggest that there exists a universal law of the form f( nu , tau ^-1, theta )=0 governing the temperature ( theta ) dependence of tau ^-1. This is independent of the phonon spectrum of the host lattice, in conflict with much recent theoretical work but in harmony with the model of Das (1957).