title: Spectral and kinetic properties of radicals derived from oxidation of quinoxalin-2-one and its methyl derivative.
abstract: The kinetics and spectral characteristics of the transients formed in the reactions of •OH and •N3 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 λmax = 470 nm assigned to the OH-adducts on the benzene ring, and a second band with λmax = 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) × 109 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 •N3 exhibits an absorption band with λmax = 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 •N3 with 3 MeQ was found to be (6.0 ± 0.5) × 109 M-1·s-1. Oxidation of 3-MeQ by •N3 and Pyr by •OH and •N3 confirms earlier spectral assignments. With the rate constant of the •OH radical with Pyr (k = 9.2 ± 0.2) × 109 M-1·s‒1, a primary distribution of the •OH attack was estimated nearly equal between benzene and pyrazin-2-one rings.
authors.affiliation: Centre of Radiation Research and Technology, Institute of Nuclear Chemistry and Technology, Dorodna 16, Warszawa 03-195, Poland. k.skotnicki@ichtj.waw.pl.
authors.forename: Konrad
authors.initials: K
authors.lastname: Skotnicki
authors.affiliation: Departamento de Quimica Orgánica y Fisicoquímica, Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile, Casilla 223, Santiago 1, Chile. jrfuente@ciq.uchile.cl.
authors.forename: Julio R
authors.initials: JR
authors.lastname: De la Fuente
authors.affiliation: Departamento de Química Orgánica, Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Correo 22, Santiago, Chile. acanetem@uc.cl.
authors.forename: Alvaro
authors.initials: A
authors.lastname: Cañete
authors.affiliation: Centre of Radiation Research and Technology, Institute of Nuclear Chemistry and Technology, Dorodna 16, Warszawa 03-195, Poland. k.bobrowski@ichtj.waw.pl.
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Bobrowski
mesh_terms: D001554:Benzene / Q000737:chemistry; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D017665:Hydroxyl Radical / Q000737:chemistry; D007700:Kinetics; D009584:Nitrogen / Q000737:chemistry; D010084:Oxidation-Reduction; D011675:Pulse Radiolysis / Q000379:methods; D011810:Quinoxalines / Q000737:chemistry*
chemical_list: D005609:Free Radicals; D011810:Quinoxalines; D017665:Hydroxyl Radical; D001554:Benzene; D009584:Nitrogen