Is the HO4-Anion a Key Species in the Aqueous-Phase Decomposition of Ozone?

The role of the HO4- anion in atmospheric chemistry and biology is a matter of debate, because it can be formed from, or be in equilibrium with, key species such as O3 + HO- or HO2 + O2-. The determination of the stability of HO4- in water therefore has the greatest relevance for better understanding the mechanism associated with oxidative cascades in aqueous solution. However, experiments are difficult to perform because of the short-lived character of this species, and in this work we have employed DFT, CCSD(T) complete basis set (CBS), MRCI/aug-cc-pVTZ, and combined quantum mechanics/molecular mechanics (QM/MM) calculations to investigate this topic. We show that the HO4- anion has a planar structure in the gas phase, with a very large HOO?OO bond length (1.823 angstrom). In contrast, HO4- adopts a nonplanar configuration in aqueous solution, with huge geometrical changes (up to 0.232 angstrom for the HOO?OO bond length) with a very small energy cost. The formation of the HO4- anion is predicted to be endergonic by 5.53 +/- 1.44 and 2.14 +/- 0.37 kcal?mol-1 with respect to the O3 + HO- and HO2 + O2- channels, respectively. Moreover, the combination of theoretical calculations with experimental free energies of solvation has allowed us to obtain accurate free energies for the main reactions involved in the aqueous decomposition of ozone. Thus, the oxygen transfer reaction (O3 + OH- ? HO2 + O2-) is endergonic by 3.39 +/- 1.80 kcal?mol-1, the electron transfer process (O3 + O2- ? O3- + O2) is exergonic by 31.53 +/- 1.05 kcal?mol-1, supporting the chain-carrier role of the superoxide ion, and the reaction O3 + HO2- ? OH + O2- + O2 is exergonic by 12.78 +/- 1.15 kcal?mol-1, which is consistent with the fact that the addition of small amounts of HO2- (through H2O2) accelerates ozone decomposition in water. The combination of our results with previously reported thermokinetic data provides some insights into the potentially important role of the HO4- anion as a key reaction intermediate.