Change with temperature of the ESR spectra of methacrylic acid radicals

In order to elucidate the structure of methacrylic acid radicals, the change with observation temperature of the ESR spectrum of free radicals trapped in solid methacrylic acid γ‐irradiated at −196°C was studied. Below −80°C, we found a 9‐line spectrum, which is similar to the ordinary 9‐line spectrum observed in irradiated poly(methacrylic acid) or poly(methyl methacrylate), but which differs in the stronger intensity of the so‐called 4‐line component. Our 9‐line spectrum changes reversibly into a 13‐line spectrum above −80°C. With broad‐line NMR measurements of methacrylic acid, it was found that there is such an unusual crystalline transition around −30°C that the line width is narrower in the lower‐temperature region (phase II) than that in the higher‐temperature region (phase I). The change of the ESR spectrum can be interpreted in terms of the exchange of the two β‐protons due to the hindered oscillation around the CαCβ bond of the single radical ···CβH2Cα(CH3)COOH if one assumes the gradual change of the hindering potential barrier caused by the crystalline transition and the lower barrier in phase II. The modified Bloch treatment gave the hindering potential barrier to be 7.2 kcal/mole in phase I and 1.5 kcal/mole in phase II. The difference between our 9‐line spectrum and the ordinary one with the very weak 4‐line component comes from the difference of the surrounding matrix.