Evidence for thermal quenching of methyl-group tunnelling rotation

The proton spin-lattice relaxation time of 4-methyl-2,6-ditertiarybutylphenol is measured over the temperature range 12-50K where the relaxation is almost entirely due to rotation of the 4-methyl group. The object is to search for a feature predicted to occur when 2 pi nuttau approximately , where nutis the frequency of coherent tunnelling rotation and tau is the time constant for randomly occurring rotations. A feature of the expected kind is found near 33K leading to a value at this temperature of nut=1.6+or-0.3 GHz. This is much less than the value of 9.4+or-0.1 GHz found by different methods in the range 4-9K. This result is in good quantitative agreement with the theoretical expectation that with increasing temperature, admixture of excited methyl-group torsional oscillator states leads to a reduction of nutthrough a process of thermal averaging.