Electron Spin Resonance and Internal Rotation of the Methyl Group in the CH3Ċ(COOH)2 Radical

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Abstract
The investigation of an x‐irradiated single crystal of methyl malonic acid by means of electron spin resonance (ESR) reveals that the two main species produced by the radiation are the CH3Ċ(COOH)2 and CH3ĊH(COOH) radicals. An analysis of the ESR spectra of CH3Ċ(COOH)2 obtained at room temperature, 77°K, and 4.2°K indicates that: (a) the three methyl protons are equivalent, and the principal values of their hyperfine coupling tensor are 75.4 (along the Ċ–CH3 bond), 68.8 and 68.6 Mc; (b) the isotropic component of the coupling tensor (and, therefore, the spin density on the methyl protons) is positive; (c) the methyl group executes nearly free rotation about the C–C bond at 4.2°K.