Electron spin resonance study of hydrogen adduct radicals generated from indole and its derivatives in γ-irradiated methanolic glasses at 77 K

Abstract

The structures of radiation-induced radicals generated from indole and a selected number of its derivatives in methanolic glasses at 77 K have been investigated by e.s.r. spectroscopy. From the spectral characteristics [tt, a_H^CH₂(2) 38.5, and a_H^CH(2) 12.0 G for indole], which are identical in almost all instances to the parent compound, it can be deduced that the radicals are formed primarily by hydrogen addition at the benzene nucleus. The α-carbon position, C-7, is favoured from the e.s.r. results obtained for selective blocking of reactive sites and by comparison with e.s.r. spectra obtained for benzene, indoline, and pyrrole in irradiated methanolic glasses and pyrrolidine irradiated in the pure state at 77 K. In the polycrystalline state only indole exhibits an e.s.r. spectrum which is indicative of the formation of a hydrogen adduct. Although there is some doubt about the homogeneity of these protic glasses containing >1M-solute it is clear that a significant contribution to the mechanism of formation of hydrogen adduct radicals comes from a process involving solvent protonation of an intermediate indole anion.