ESR and proton ENDOR studies in urea oxalic acid (2:1) single crystal: Radical transformation

Abstract

A radical transformation has been investigated in a single crystal of urea oxalic acid (2:1) irradiated at room temperature. ENDOR studies (∼−160 °C) have been performed on both the initial and final radicals. The initial species has been assigned to a protonated anion radical. The results indicated that a common proton was involved in the protonation of the anion and in the subsequent transformation to R–ĊHOH. Two important differences in relation to the earlier studies on the protonated anions were noted. The maximum dipolar coupling values (Bmax) for the O–Hβ protons are smaller than normal and the proton transfer is not across the hydrogen bridge. A qualitative estimation of the spin densities indicated delocalization and the possible causes for small values of Bmax are discussed. A mechanism proposed earlier is invoked to understand the radical transformation.