Hydrogen Isotope Effects and Mechanism of Aqueous Ozone and Peroxone Decompositions

Abstract

Hydrogen peroxide exalts the reactivity of aqueous ozone by reasons that remain obscure. Should H_2O_2 enhance free radical production, as it is generally believed, a chain mechanism propagated by (·OH/·O_2^-) species would account for O_3 decomposition rates in neat H_2O, ^HR_(-O_3), and in peroxone (O_3 + H_2O_2) solutions, ^(HP)R_(-O_3). We found, however, that:  (1) the radical mechanism correctly predicts H^R_(-O_3) but vastly overestimates ^(HP)R_(-O_3), (2) solvent deuteration experiments preclude radical products from the (O_3 + HO_2^-) reaction. The modest kinetic isotope effect (KIE) we measure in H_2O/D_2O:  ^HR_(-O_3)/^DR_(-O_3) = 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:  ^(HP)R_(-O_3)/^(DP)R_(-O_3) = 19.6 ± 4.0, is due to an elementary (O_3 + HO_2^-) reaction involving H−O_2^- bond cleavage. Since the KIE for the hypothetical H-atom transfer:  O_3 + HO_2^- →(2ℏ) HO^3· + ·O_2^-, would emerge as a KIE^(1/2) 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:  O_3 + HO_2^- HO_3^- + O_2. HO_3^-/H_2O_3 may be the bactericidal trioxide recently identified in the antibody-catalyzed addition of O_2(^1Δ_g) to H_2O.