Nuclear spin lattice relaxation and activation energies of tunnelling methyl groups

P A Beckmann, S Clough
1977 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
more » ... ation time from 6 to 50 K 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 firstexcited methyl group torsional states, as computed from the known tunnelling frequencies asuming a three-fold hindering potential. At temperatures below 15 K the observed activation energy is only about half this value.
doi:10.1088/0022-3719/10/9/002 fatcat:3gqbily7kjfgtmppbxpyvhjryy