The temperature dependence of methyl tunnelling motion in three acetates

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Abstract
High-resolution inelastic neutron scattering is used to study the excitations of the three proton spin symmetry species of the tunnelling CH3group in sodium, zinc and ammonium acetates at temperatures below 30K. The inelastic and quasi-elastic peaks are found to have similar widths. This shows that the thermal excitations of the three species due to fluctuations of the hindering barrier are statistically independent. The methyl tunnelling splittings are obtained from the positions of the inelastic peaks at low temperature. The tunnel splitting decreases in each case with increasing temperature. The observed splitting is in approximate agreement with a thermally weighted average of the splittings of the ground and first excited torsional states.