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

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Skotnicki, Konrad ; Fuente, Julio De la ; Cañete, Alvaro ; Bobrowski, Krzysztof (2014)
  • Publisher: Multidisciplinary Digital Publishing Institute
  • Journal: Molecules (issn: 1420-3049)
  • Related identifiers: doi: 10.3390/molecules191119152
  • Subject: pulse radiolysis | radicals | Organic chemistry | QD241-441 | quinoxalin-2-ones | azide radicals | hydroxyl radicals

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.
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