NUCLEAR-SPIN-LATTICE RELAXATION OF METHYL-GROUPS VIA MULTIPHONON PROCESSES

The empirical correlation between the logarithm of tunneling splittings and the temperature at which the spin-lattice relaxation time is minimum for methyl group in different molecular crystals is explained successfully by taking mulitiphonon processes into account. We show that one-phonon transitions dominate in the low barrier limit. However, the intemediate barrier range and high barrier limit, it is necessary to include mulitphonon processes. We also show that the empirical correlation depends only logarithmically on the details of the phonon bath.