Electron Spin Resonance Study of Radicals Produced by One-Electron Reduction of Pyridines

Radicals produced in irradiated aqueous solutions of carboxy and N-methyl derivatives of pyridine have been studied by the in situ radiolysis steady-state ESR technique. All solutions contained other efficient OH scavengers so that only radicals produced by reaction of the pyridines with <tex-math>$e_{{\rm aq}}^{-}$</tex-math> have been observed. The electron adducts initially produced were found to undergo protonation on the ring nitrogen (if not methylated) to produce pyridinyl radicals. Protonation occurred rapidly both in neutral and strongly alkaline solutions, which indicates that a molecule of water rather than H⁺ is involved in the process. The pK of the ring NH group of the pyridinyl radicals if above 13 in all cases examined. The ortho and para carboxy derivatives were found to produce pyridinyl radicals which are long-lived as compared to the unsubstituted or meta carboxy compounds. The higher stability is probably a result of increased conjugation in the case of ortho and para derivatives, which is also expressed by their effect on the spin density on the ring.