Electron Spin Resonance Study of Radiation Damage in Single Crystals of dl-Aspartic Acid

Single crystals of dl-aspartic acid were examined by the electron spin resonance method after being damaged at room temperature by x irradiation. Analyses of the resulting spectra suggest that the mechanism of the damage is the removal of the amino group, resulting in the stably trapped radical H –HOOC–C–C lim ·–COOH. – – H HThe three protons nearest to the unpaired electron were nonequivalent. The observed values of the isotropic π-electron—π-proton hyperfine couplings were 115±2 and 18±2 Mc. Two nonequivalent radical sites were detected. The values of the two nonequivalent anisotropic π-electron—σ-proton hyperfine interactions were observed to be 90±3 and 50±3 Mc along one axis. Interpretation of the π-proton couplings indicates that one of the methylene C–H bonds lies parallel to the π orbital of the unpaired electron, whereas the other C–H bond in the methylene group makes an angle of 113° with this direction. Comparison of the spectra from the two nonequivalent radical sites suggests that the angle between these is 60±10°. These conclusions were confirmed by similar studies of single crystals of deuterated dl-aspartic acid.