Genuine Pyrimidine Radical Cations Generated by Radiation-Induced Electron Transfer to Butyl Chloride or Acetone Parent Ions

Radical cations of multiple methylated uracils and thymines were generated by electron transfer to parent ions of solvents of low polarity such as n -butyl chloride and acetone. Pulse radiolysis resulted in the transient optical absorption spectra of the pyrimidine radical cations (Py •+ ) which exhibit bands generally peaking around λ = 400 nm and in acetone showing an additional band in the range of λ max = 500−560 nm. The identification of the solute radical cations is based on the direct observation of the formation of Py •+ by electron transfer ( k = (1−2) × 10 10 M -1 s -1 ), the nucleophilic reaction with the counterion (e.g., Cl - ), the relatively low reactivity with oxygen even for the fully methylated nitrogen groups ( k < 2 × 10 8 M -1 s -1 ), and the observation of a subsequent electron transfer from triphenylamine to the pyrimidine radical cations proceeding in a diffusion-controlled manner. The solvent dependence of the spectral shape of Py •+ was also studied in acetone/ n -butyl chloride mixtures and was found to depend on the polarity of the surroundings. This phenomenon is explained in terms of a lactam−lactim-like tautomerism of the transient involving the whole plane molecule. Quantum chemical calculations support the experimental findings by explaining the spectral structure and enable the formulation of the lactims as either 1,4- or 3,4-localized.