Restricted Motion of β Protons of the HOOC–CH2–CH–COOH Radical Trapped in Urea Crystals

The ESR spectrum of the HOOC–CH2–CH–COOH radical, formed by γ-ray irradiation of a mixed crystal of urea—fumaric acid, was recorded at different temperatures. Analysis of the spectra shows that the radical undergoes some restricted torsion about the Cα–Cβ bond. A value of ∼2 kcal/mole was found for the energy barrier opposing this motion.