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Abstract
The coupled relaxation of the Zeeman and tunnel systems of methyl groups has been studied by N.M.R. spectroscopy in powdered pentamethyl-benzene at temperatures below 15 K, in the magnetic field range 0-2 T. The observed rapidity, saturation and recovery of the resonance confirm theoretical predictions of Nijman, Sprik and Trappeniers [12] for (X-H4) substances, and the equivalence of their theory adapted by us to methyl groups using the simplified relaxation model of Emid and Wind [4] is established. Reasons for the semi-quantitative agreement between theory and experiment are discussed.