Torsional Relaxation of Methyl-Group Nuclear-Spin-Symmetry Species

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.