Optical-Absorption Spectra of Ketyl Radicals and Radical Anions of Some Pyrimidines

The pulse radiolysis of aqueous solutions of various pyrimidines has been studied under conditions where only solvated electrons are made to react with them. Transient optical-absorption spectra in the uv range have been obtained which provide information on the site of attack of eaq− on the pyrimidine molecule. These spectra have been identified as the ketyl radicals of the C2 and C4 carbonyls, based on examination of substituted pyrimidine derivatives and making use of the keto–enol tautomerism which these molecules undergo. At pH5.0eaq− add at both the C2 and C4 positions, and the intermediates are rapidly protonated to give the corresponding ketyl radicals. The ketyl radicals dissociate to the corresponding ketyl radical anions with a pK ∼ 7.0–7.5. In alkaline solutions, on ionization of one of the chromophores the ketyl radical anion of the other nondissociated carbonyl is formed. The ketyl radicals were found to have an extinction coefficient of ∼1000 M−1·cm−1 and to decay by a second-order process with 2k ∼ 109 M−1·sec−1.