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 |
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| Authors: | Gábor Merényi Prof Dr Johan Lind Prof Dr Sergej Naumov Dr Clemens von Sonntag Prof Dr |
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| Institution: | 1. School of Chemistry, Nuclear Chemistry, The Royal Institute of Technology, 10044 Stockholm (Sweden), Fax: (+46)?8‐7908722;2. Leibniz‐Institut für Oberfl?chenmodifizierung (IOM), Permoserstrasse 15, 04303 Leipzig (Germany);3. Max‐Planck‐Institut für Bioanorganische Chemie, Stiftstrasse 34–36, 45413 Mülheim an der Ruhr (Germany);4. Universit?t Dortmund, Fachbereich Bio‐ und Chemieingenieurwesen, Lehrstuhl Umwelttechnik, Emil‐Figge‐Strasse 70, 44227 Dortmund (Germany), Fax: (+49)?231‐7556194 |
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| Abstract: | The reaction of OH? with O3 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?+O3→HO2?+O2 and that .OH was generated in the subsequent reaction(s) of HO2? with O3 (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 O2 in its excited singlet state (1O2, O2(1Δg)); this state lies 95.5 kJ mol?1 above the triplet ground state (3O2, O2(3Σg?)). 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 O3 to form an adduct (OH?+O3→HO4?; ΔG=3.5 kJ mol?1) is a much better candidate for the rate‐determining step as compared with the significantly more endergonic O transfer (ΔG=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?+1O2 is spin allowed, but markedly endergonic (ΔG=23.2 kJ mol?1). HO4?→HO2?+3O2 is spin forbidden (ΔG=?73.3 kJ mol?1). The decay into radicals, HO4?→HO2.+O2.?, is spin allowed and less endergonic (ΔG=14.8 kJ mol?1) than HO4?→HO2?+1O2. It is thus HO4?→HO2.+O2.? by which HO4? decays. It is noted that a large contribution of the reverse of this reaction, HO2.+O2.?→HO4?, followed by HO4?→HO2?+3O2, now explains why the measured rate of the bimolecular decay of HO2. and O2.? into HO2?+O2 (k=1×108 M ?1 s?1) is below diffusion controlled. Because k for the process HO4?→HO2.+O2.? is much larger than k for the reverse of OH?+O3→HO4?, the forward reaction OH?+O3→HO4? is practically irreversible. |
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| Keywords: | density functional calculations kinetics ozone radicals reactive intermediates |
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