Lesko Timothy M, Colussi A J, Hoffmann Michael R
W. M. Keck Laboratories, California Institute of Technology, Pasadena, California 91125, USA.
J Am Chem Soc. 2004 Apr 7;126(13):4432-6. doi: 10.1021/ja038907v.
Hydrogen peroxide exalts the reactivity of aqueous ozone by reasons that remain obscure. Should H2O2 enhance free radical production, as it is generally believed, a chain mechanism propagated by (.OH/.O2-) species would account for O3 decomposition rates in neat H2O, HR-O3, and in peroxone (O3 + H2O2) solutions, HPR-O3. We found, however, that: (1) the radical mechanism correctly predicts HR-O3 but vastly overestimates HPR-O3, (2) solvent deuteration experiments preclude radical products from the (O3 + HO2-) reaction. The modest kinetic isotope effect (KIE) we measure in H2O/D2O: HR-O3/DR-O3 = 1.5 +/- 0.3, is compatible with a chain process driven by electron- and/or O-atom transfer processes. But the large KIE found in peroxone: HPR-O3/DPR-O3 = 19.6 +/- 4.0, is due to an elementary (O3 + HO2-) reaction involving H-O2- bond cleavage. Since the KIE for the hypothetical H-atom transfer: O3 + HO2- HO3. +.O2-, would emerge as a KIE1/2 factor in the rates of the ensuing radical chain, the magnitude of the observed KIE must be associated with the hydride transfer reaction that yields a diamagnetic species: O3 + HO2- HO3- + O2. HO3-/H2O3 may be the bactericidal trioxide recently identified in the antibody-catalyzed addition of O2(1Deltag) to H2O.
过氧化氢会提高水中臭氧的反应活性，但其原因仍不清楚。如果过氧化氢像人们普遍认为的那样增强自由基的产生，那么由（·OH/·O₂⁻）物种传播的链式反应机制将解释纯水中、HR - O₃以及过氧单硫酸盐（O₃ + H₂O₂）溶液HPR - O₃中的O₃分解速率。然而，我们发现：（1）自由基机制正确预测了HR - O₃，但大大高估了HPR - O₃；（2）溶剂氘代实验排除了（O₃ + HO₂⁻）反应产生的自由基产物。我们在H₂O/D₂O中测得的适度动力学同位素效应（KIE）：HR - O₃/DR - O₃ = 1.5 ± 0.3，与由电子和/或氧原子转移过程驱动的链式反应过程相符。但在过氧单硫酸盐中发现的较大KIE：HPR - O₃/DPR - O₃ = 19.6 ± 4.0，是由于涉及H - O₂⁻键断裂的基元（O₃ + HO₂⁻）反应。由于假设的氢原子转移O₃ + HO₂⁻→HO₃· + ·O₂⁻的KIE会在随后的自由基链反应速率中以KIE¹/₂因子出现，所以观察到的KIE大小必定与产生抗磁性物种的氢化物转移反应有关：O₃ + HO₂⁻→HO₃⁻ + O₂。HO₃⁻/H₂O₃可能是最近在抗体催化O₂(¹Δg)与H₂O加成反应中鉴定出的杀菌三氧化物。
