The lifetime of the first excited torsional state of a hindered methyl group of E proton-spin symmetry is found to be much less than the A-species lifetime. This is explained by a symmetric hindering potential, amplitude modulated by phonons, which also accounts for the observed widths of torsional ground-state tunneling transitions. The observations invalidate Allen's model for tunneling-splitting temperature dependence.
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Clough et al. (1979) studied this question.
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