Radiation-induced reduction of quinoxalin-2-one derivatives in aqueous solutions

• Rate constants k  (eaq− +Q/3−MeQ) have been determined at 25 °C at two pHs (7 and 13).
• Equilibrium constants (pKa≥13.5) for the protropic equilibria have been provided.
• Spectral characteristics for Q∙−/3-MeQ∙− and QH∙/3-MeQH∙ have been determined.
• The most stable hydrogenated radicals (QH∙ and 3-MeQH∙) have been defined.

Quinoxaline-2-one derivatives have been proposed as potential drugs in treatments of various diseases since some of them showed a variety of pharmacological properties. The kinetics and spectral characteristics of the transients formed in the reactions of hydrated electrons (eaq−) with quinoxalin-2-(1H)-one (Q) and its methyl derivative, 3-methyl quinoxalin-2-(1H  )-one (3-MeQ) were studied by pulse radiolysis in aqueous solutions at pH ranging from 5 to 14. The transient absorption spectra recorded in the reactions of (eaq−) with Q and 3-MeQ at pH 7 consisted of a broad, almost flat band in the range 390–450 nm and were assigned to the respective protonated radical anions (QH∙/3-MeQH∙) at N4 atom in a pyrazin-2-one ring. On the other hand, the transient absorption spectra recorded in the reactions of (eaq−) with Q and 3-MeQ at pH 13 are characterized by a broad band with a much better pronounced maximum at λmax=390 nm and higher intensity (in comparison to that at pH 7) and were assigned to the respective radical anions (Q∙−/3-MeQ∙−). Both forms are involved in the prototropic equilibrium with the pKa located at pH≥13.5. The rate constants of the reactions of (eaq−) with Q and 3-MeQ were found to be at pH 7 (2.6±0.1)×1010 M−1 s−1 and (2.1±0.1)×1010 M−1 s−1 and at pH 13 (1.6±0.1)×1010 M−1 s−1 and (1.3±0.1)×1010 M−1 s−1, respectively. Semi-empirical quantum mechanical calculations reproduce fairly well the spectral features of the experimental absorption spectra and show that protonated radical anions at nitrogen atom (N4) in both molecules are the most stable hydrogenated radicals.