Final Published Version
Journal of Physics C: Solid State Physics
The coupling of lattice vibrations with the rotational motion of hindered methyl groups is considered. It is shown that the apparent activation energy for methyl group rotation is determined at relatively high temperatures by the methyl group torsional energy splittings, but at low temperatures it depends on the frequencies of short-wavelength lattice modes which couple relatively strongly with methyl-group rotation. The temperature dependence of the proton spin lattice relaxation time from 6 to 50K is reported for three samples containing methyl groups whose tunnelling frequencies have previously been measured very precisely. They are 4-methyl-2,6-ditertiarybutylphenol (MDBP) and two deuterated derivatives. The activation energy measured at temperatures between 18 and 50K is in good agreement with the splitting between ground and first-excited methyl group torsional states, as computed from the known tunnelling frequencies assuming a three-fold hindering potential. At temperatures below 15K the observed activation energy is only about half this value.
Beckmann, P. A. and S. Clough. 1977. "Nuclear spin-lattice relaxation and activation energies of tunnelling methyl groups." Journal of Physics C: Solid State Physics 10.9 : L231-L236.