Electron spin resonance evidence of the photoionization and photoisomerization of free radicals from γ and ultraviolet irradiated pyridinium cations

Abstract

The analysis of the e.s.r. spectra of u.v. illuminated (250 < λ < 400 nm) pyridinium cation and of several of its derivatives in an HCl + H₂O glassy matrix shows the presence of an allyl radical (IIId). [graphic omitted]

This radical is also formed under u.v. illumination of the azacyclohexadienyl radical (I) produced by γ irradiation of glassy solutions of pyridinium cations. Radical (I) is not the direct precursor of radical (IIId). Evidence is presented to support the sequence: [graphic omitted] e^–+ pyridinium cation → pyridinyl radical → azaprismane radical → allyl radical (IIId).

The transient formation of an ammonium like azaprismane radical is suggested by the photolysis of some methyl substituted pyridines which indicates an important rearrangement of the carbon backbone of the pyridinyl radical leading to the radical (IIId).