Motional Averaging of Methyl-Group ESR Hyperfine Structure

A model for the modulation of the methyl-group rotational tunneling frequency at low temperatures, which was previously introduced to account for proton spin-lattice relaxation, is here shown to predict hitherto unexpected features in methyl-group ESR hyperfine structure connected with the change with rising temperature from eight lines to four. Two experiments are described which confirm the predictions.