Electron Spin Resonance Study of Radiation Damage in Single Crystals of Urea Compounds

Abstract

Single crystals of urea compounds were examined by 3‐cm electron spin resonance techniques after x and γ irradiation. All experiments were conducted at room temperature. No resonances were observed in urea and thiourea. This is interpreted to indicate that the lifetimes of any resulting unstable species must be less than 30 min. at room temperature. However, methylurea and ethylurea showed strong, highly stable spectra centered at g≃2.00. Analyses of these spectra suggest that the trapped radicals [Complex chemical formula] are formed from methylurea and ethylurea, respectively, by the loss of a hydrogen atom. In both cases all radicals in the unit cell were considered to be magnetically equivalent. Calculated hyperfine intervals agree favorably with those observed. The spin density of the unpaired electron at the adjacent N–H group is believed less than 0.002, since the only apparent effect of this group was a broadening of the hyperfine lines. In methylurea the ·C–H₂ group was found to have an H–C–H angle of 117°. The principal components of the diagonal electron‐nuclear coupling dyadic were 10, 22, and 32 gauss. In ethylurea the ·C–H bond was observed to lie along a crystallographic axis. The principal components were 10, 21, and 24 gauss. In this case the isotropic coupling constant was 18 gauss.