Electron–electron double resonance study of coherent and random rotational motion of methyl groups

Abstract

Electron–electron double resonance(ELDOR) spectra are reported for the free radical CH3C̊ (COOH)2 in x‐irradiated crystals of methylmalonic acid over the temperature range 10–50 K. At low temperatures tunneling sidebands are observed corresponding to simultaneous flips of the electron spin and changes in the tunneling rotation state of the methyl group. The previous assignment of weak satellite lines of the electron spin resonance(ESR) spectrum to these transitions is unequivocally confirmed, and sidebands obscured in the ESR spectrum are clearly observed in the ELDOR spectrum with the expected frequencies and intensities. The temperature dependence of the frequencies confirms the previous observation of the temperature dependence of the frequency of tunneling rotation of the methyl group. A sequence of changes with rising temperature in the relative amplitudes of lines in the ELDOR spectra is accounted for in a qualitative way as a consequence of the changing rates of several cross relaxation processes, all of which arise as a consequence of the modulation of the electron–proton hyperfine interaction by thermally activated rotation of the methyl group about its symmetry axis.