The gas phase reaction of singlet dioxygen with water: A water-catalyzed mechanism

Stimulated by the recent surprising results from Wentworth et al. [Wentworth, A. D., Jones, L. H., Wentworth, P., Janda, K. D. & Lerner, R. A. (2000) Proc. Natl. Acad. Sci. USA 97, 10930-10935] that Abs efficiently catalyze the conversion of molecular singlet oxygen (102) plus water to hydrogen peroxide (HOOH), we used quantum chemical methods (B3LYP density functional theory) to delineate the most plausible mechanisms for the observed efficient conversion of water to HOOH. We find two reasonable pathways. In Pathway I, (i) H₂O catalyzes the reaction of ¹O₂ with a second water to form HOOOH; (ii) two HOOOH form a dimer, which rearranges to form the HOO-HOOO + H₂O complex; (iii) HOO-HOOO rearranges to HOOH-OOO, which subsequently reacts with H₂O to form H₂O₄ + HOOH; and (iv) H₂O₄ rearranges to the cyclic dimer (HO₂)₂, which in turn forms HOOH plus ¹O₂ or ³O₂. Pathway II differs in that step ii is replaced with the reaction between HOOOH and ¹O₂, leading to the formation of HOO-HOOO. This then proceeds to similar products, For a system with ¹⁸O H₂O, Pathway I leads to a 2.2:1 ratio of ¹⁶O:¹⁸O in the product HOOH, whereas Pathway II leads to 3:1. These ratios are in good agreement with the 2.2:1 ratio observed in isotope experiments by Wentworth et al. These mechanisms lead to two HOOH per initial ¹O₂ or one, depending on whether the product of step iv is ¹O₂ or ³O₂, in good agreement with the experimental result of 2.0. In addition to the Ab-induced reactions, the hydrogen polyoxides (H₂O₃ and H₂O₄) formed in these mechanisms and their decomposition product polyoxide radicals (HO₂, HO₃) may play a role in combustion, explosions, atmospheric chemistry, and the radiation chemistry in aqueous systems.