Proton relaxation and hyperfine structure due to tunnelling methyl groups

Experiments are reported in support of a previous theoretical model for the low temperature tunnelling rotation of hindered methyl groups. It is demonstrated that the low temperature change in the electron spin resonance hyperfine structure of free radicals containing tunnelling methyl groups and the low temperature peak in the proton spin lattice relaxation rate of materials containing tunnelling methyl groups are two manifestations of a single process. The two kinds of experiment have been conducted on essentially the same system from the point of view of the dynamics of the methyl group. The peak in the relaxation rate and the hyperfine structure change both occur near 14 K and activation energies extracted from the two experiments are in substantial agreement. Because the theoretical model gives a frequency spectrum for the methyl group rotational motion which departs very considerably from the classical Lorentz shape, the value of the maximum relaxation rate is only one third of the classical value. The experimental value supports this smaller value.