Tunnelling sidebands of methyl group hyperfine structure

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Abstract
The authors report the first observation of certain weak transitions which occur as sidebands of electron spin resonance hyperfine structure due to the protons of a methyl group executing tunnelling rotation at 4K. The transitions involve an electron spin flip and a simultaneous change in the torsional oscillator tunneling state of the methyl group. The transitions were observed in free radicals generated by gamma irradiation of crystals of methyl malonic acid. They enable the tunnelling splitting to be measured to 2% and the minimum energy orientation of the methyl group to be located to 4 degrees . The transitions were found after their position and intensity had been predicted from electron nuclear double resonance measurements. They were positively identified through their angular dependence and the fact that they disappear at higher temperatures.