A study of the temperature dependence of a methyl tunnelling spectrum by inelastic neutron scattering

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Abstract
High-resolution inelastic neutron scattering is used to study the tunnelling rotation spectrum of the 4-methyl group in MDBP (4-methyl-2, 6-ditertiarybutylphenol) over a temperature range 4-30K. The spectrum shows the approximately Lorentzian inelastic and quasi-elastic peaks and the measured parameters are the two widths and the energy transfer of the inelastic peak. At low temperatures the latter is equal to the tunnelling splitting of the ground torsional state, but at higher temperatures the energy of the inelastic peak also depends on the tunnelling splitting of excited torsional states of the methyl group. The observed temperature dependence is used to test a stochastic model for the dynamic averaging of the splittings of the thermally occupied states. The results show two different rotational processes are dominant above and below 15K. These are assigned to a torsional excitation process and a Raman process involving only a virtual torsional excitation.