Electron spin resonance of γ-irradiated malonic acid

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Abstract
The electron spin resonance spectrum of a γ-irradiated single crystal of malonic acid indicates the presence of at least two chemically distinct radicals. The most stable radical is ĊH(CO2H)2, but the present paper primarily concerns the structure and orientation of the radical ĊH2(CO2H). The anisotropy of the spectra of this radical with respect to the direction of the main magnetic field H shows that the free radical carbon atom has gone over to planar sp 2 hybridization, the HĊH angle being 116°±5°. Since the two ends of the undamaged malonic acid molecule are distinguishable in the crystal, two sites were expected for the radical ĊH2(CO2H). It appears, however, that approximately 80 per cent of the radicals occupy one site, and only about 20 per cent the other. Comparison of the orientation of the main ĊH2(CO2H) radical with that of the undamaged molecule indicates that in the radical the plane of the ĊH2 group is approximately perpendicular to the plane of the (CO2H) group. The spectra of the less predominant ĊH2(CO2H) radicals were too weak and too overlapped for detailed analysis.

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