Spectral Probe for Electron Transfer and Addition Reactions of Azide Radicals with Substituted Quinoxalin-2-Ones in Aqueous Solutions

The azide radical (N-3(?)) is one of the most important one-electron oxidants used extensively in radiation chemistry studies involving molecules of biological significance. Generally, it was assumed that N-3(?) reacts in aqueous solutions only by electron transfer. However, there were several reports indicating the possibility of N-3(?) addition in aqueous solutions to organic compounds containing double bonds. The main purpose of this study was to find an experimental approach that allows a clear assignment of the nature of obtained products either to its one-electron oxidation or its addition products. Radiolysis of water provides a convenient source of one-electron oxidizing radicals characterized by a very broad range of reduction potentials. Two inorganic radicals (SO4?-, CO3?-) and Tl2+ ions with the reduction potentials higher, and one radical (SCN)(2)(?-) with the reduction potential slightly lower than the reduction potential of N-3(?) were selected as dominant electron-acceptors. Transient absorption spectra formed in their reactions with a series of quinoxalin-2-one derivatives were confronted with absorption spectra formed from reactions of N-3(?) with the same series of compounds. Cases, in which the absorption spectra formed in reactions involving N-3(?) differ from the absorption spectra formed in the reactions involving other one-electron oxidants, strongly indicate that N-3(?) is involved in the other reaction channel such as addition to double bonds. Moreover, it was shown that high-rate constants of reactions of N-3(?) with quinoxalin-2-ones do not ultimately prove that they are electron transfer reactions. The optimized structures of the radical cations (7-R-3-MeQ)(?+), radicals (7-R-3-MeQ)(?) and N-3(?) adducts at the C2 carbon atom in pyrazine moiety and their absorption spectra are reasonably well reproduced by density functional theory quantum mechanics calculations employing the omega B97XD functional combined with the Dunning's aug-cc-pVTZ correlation-consistent polarized basis sets augmented with diffuse functions.