The Reaction of Ozone with the Hydroxide Ion: Mechanistic Considerations Based on Thermokinetic and Quantum Chemical Calculations and the Role of HO 4 − in Superoxide Dismutation

The reaction of OH − with O 3 eventually leads to the formation of . OH radicals. In the original mechanistic concept (J. Staehelin, J. Hoigné, Environ. Sci. Technol. 1982 , 16 , 676–681), it was suggested that the first step occurred by O transfer: OH − +O 3 →HO 2 − +O 2 and that . OH was generated in the subsequent reaction(s) of HO 2 − with O 3 (the peroxone process). This mechanistic concept has now been revised on the basis of thermokinetic and quantum chemical calculations. A one‐step O transfer such as that mentioned above would require the release of O 2 in its excited singlet state ( 1 O 2 , O 2 ( 1 Δ g )); this state lies 95.5 kJ mol −1 above the triplet ground state ( 3 O 2 , O 2 ( 3 Σ g − )). The low experimental rate constant of 70  M −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 →HO 4 − ; Δ G =3.5 kJ mol −1 ) is a much better candidate for the rate‐determining step as compared with the significantly more endergonic O transfer (Δ 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 HO 4 − have been probed: HO 4 − →HO 2 − + 1 O 2 is spin allowed, but markedly endergonic (Δ G =23.2 kJ mol −1 ). HO 4 − →HO 2 − + 3 O 2 is spin forbidden (Δ G =−73.3 kJ mol −1 ). The decay into radicals, HO 4 − →HO 2 . +O 2 .− , is spin allowed and less endergonic (Δ G =14.8 kJ mol −1 ) than HO 4 − →HO 2 − + 1 O 2 . It is thus HO 4 − →HO 2 . +O 2 .− by which HO 4 − decays. It is noted that a large contribution of the reverse of this reaction, HO 2 . +O 2 .− →HO 4 − , followed by HO 4 − →HO 2 − + 3 O 2 , now explains why the measured rate of the bimolecular decay of HO 2 . and O 2 .− into HO 2 − +O 2 ( k =1×10 8   M −1  s −1 ) is below diffusion controlled. Because k for the process HO 4 − →HO 2 . +O 2 .− is much larger than k for the reverse of OH − +O 3 →HO 4 − , the forward reaction OH − +O 3 →HO 4 − is practically irreversible.