Spectral and Kinetic Properties of Radicals Derived from Oxidation of Quinoxalin-2-One and Its Methyl Derivative

The kinetics and spectral characteristics of the transients formed in the reactions of (•)OH and (•)N(3) with quinoxalin-2(1H)-one (Q), its methyl derivative, 3-methylquinoxalin-2(1H)-one (3-MeQ) and pyrazin-2-one (Pyr) were studied by pulse radiolysis in aqueous solutions at pH 7. The transient absorption spectra recorded in the reactions of (•)OH with Q and 3-MeQ consisted of an absorption band with λ(max) = 470 nm assigned to the OH-adducts on the benzene ring, and a second band with λ(max) = 390 nm (for Q) and 370 nm (for 3-MeQ) assigned, inter alia, to the N-centered radicals on a pyrazin-2-one ring. The rate constants of the reactions of (•)OH with Q and 3-MeQ were found to be in the interval (5.9–9.7) × 10(9) M(–1)·s(–1) and were assigned to their addition to benzene and pyrazin-2-one rings and H-abstraction from the pyrazin-2-one nitrogen. In turn, the transient absorption spectrum observed in the reaction of (•)N(3) exhibits an absorption band with λ(max) = 350 nm. This absorption was assigned to the N-centered radical on the Pyr ring formed after deprotonation of the respective radical cation resulting from one-electron oxidation of 3-MeQ. The rate constant of the reaction of (•)N(3) with 3 MeQ was found to be (6.0 ± 0.5) × 10(9) M(–1)·s(–1). Oxidation of 3-MeQ by (•)N(3) and Pyr by (•)OH and (•)N(3) confirms earlier spectral assignments. With the rate constant of the (•)OH radical with Pyr (k = 9.2 ± 0.2) × 10(9) M(–1)·s(‒1), a primary distribution of the (•)OH attack was estimated nearly equal between benzene and pyrazin-2-one rings.