Tunnelling rotation rate of a weakly hindered methyl group (e.p.r. and ENDOR measurements)

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Abstract
Electron spin resonance spectra and electron nuclear double resonance measurements are presented for free radicals with the structure CH3COHCOO- formed by gamma irradiation of crystals of lithium lactate. The splitting of the ground state due to the tunnelling rotation of the methyl group is found from the temperature dependence of the populations of the split levels which are reflected in the relative heights of constituent lines of the hyperfine structure. The amplitude of the zero-point torsional oscillation is measured from the relative magnitude of hyperfine coupling tensors and the height of the hindering barrier is deduced from the temperature dependence of the ESR spectrum. All the measurements are consistent with a tunnelling splitting of 22+or-5 GHz.