Electron spin resonance and torsional motion of the β protons of the HOOC—CH2—C(OH)—COOH radical

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Abstract
The e.s.r. spectrum of the free radical HOOC—CH2—Ċ(OH)COOH formed by γ-ray irradiation of single crystals of ammonium malate monohydrate has been recorded at different temperatures. The change in the β proton coupling constants with the temperature is discussed in terms of a torsional motion about the Cα—Cβ single bond. The C13 splitting and the hydroxyl proton splitting are correlated to the conformation of the hydroxylic group, which results to be about 20° out of the radical plane. The temperature dependence of the coupling constant is consistent with the presence of strong intermolecular forces which hinder the motion. They probably arise from hydrogen bonds in the solid state.

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