The Reaction of Ozone with the Hydroxide Ion: Mechanistic Considerations Based on Thermokinetic and Quantum Chemical Calculations and the Role of HO4- in Superoxide Dismutation

The reaction of OH- with O-3 eventually leads to the formation of center dot OH radicals. In the original mechanistic concept (J. Staehelin, J. Hoigne, Environ. Sci. Technol. 1982, 16, 676-681) it was suggested that the first step occurred by O transfer: OH- + O-3 -> HO2- + O-2 and that center dot OH was generated in the subsequent reaction(s) of HO2- with O-3 (the peroxone process). This mechanistic concept has now been revised on the basis of thermokinetic and quantum chemical calculations. A onestep O transfer such as that mentioned above would require the release of O-2 in its excited singlet state (O-1(2), O-2-((1)Delta(g))); this state lies 95.5 kJ mol(-1) above the triplet ground state ((3)Sigma(-)(g))). The low experimental rate constant of 70m(-1) s(-1) is not incompatible with such a reaction. However, according to our calculations, the reaction of OH- with O-3 to form an adduct (OH- + O-3 -> HO4-; Delta G = 3.5 kJ mol(-1)) is a much better candidate for the rate-determining step as compared with the significantiv more endergonic O transfer (Delta G=26.7 kJ mol(-1)). Hence, we favor this reaction; all the more so as numerous precedents of similar ozone adduct formation are known in the literature. Three potential decay routes of the adduct HO4- have been probed: HO4- -> HO2- + O-1(2) is spin allowed, but markedly endergonic (Delta G=23.2 kJ mol(-1)). HO4- -> HO2- + O-3(2) is spin forbidden (Delta G = -73.3 kJ mol(-1)). The decay into radicals, HO4- -> HO2 center dot + O-2(center dot-), is spin allowed and less endergonic (Delta G=14.8 kJ mol(-1)) than HO4- -> HO2- + O-1(2). It is thus HO4- -> HO2 center dot + O-2(center dot-) by which HO4- decays. It is noted that a large contribution of the reverse of this reaction, HO2 center dot + O-2(center dot-)-> HO4-, followed by HO4- -> HO2- + O-3(2), now explains why the measured rate of the bimolecular decay of HO2 center dot and O-2(center dot-) into HO2- + O-2 (k=1x10(8) m(-1) s(-1)) is below diffusion controlled. Because k for the process HO4- -> HO2 center dot + O-2(center dot-) is much larger than k for the reverse of OH- + O-3 -> HO4-, the forward reaction OH- + O-3 -> HO4- is practically irreversible.