The approach to classical dynamics of reorienting methyl groups

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Abstract
The authors have studied the approach towards classical behaviour of a dynamical system which exhibits strong quantum effects at low temperature. By combining nuclear magnetic relaxation and inelastic neutron scattering data, the rate of thermally activated methyl-group reorientation is measured from 10 to 65K for the 4-methyl group of MDBP (4-methyl-2,6-ditertiarybutylphenol). At low temperatures reorientation occurs by thermally assisted quantum tunnelling processes. The reorientation rate converges towards the predictions of classical theory with increasing temperature, but still deviates significantly at 65K. It appears that the classical theory may be used without important errors only at temperatures for which kT exceeds the methyl torsion splitting. In this high-temperature range a simple classical calculation of the reorientation rate is within a factor of 2 or 3 of the measured value.