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 ((1)O(2)) 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) H(2)O catalyzes the reaction of (1)O(2) with a second water to form HOOOH; (ii) two HOOOH form a dimer, which rearranges to form the HOO-HOOO + H(2)O complex; (iii) HOO-HOOO rearranges to HOOH-OOO, which subsequently reacts with H(2)O to form H(2)O(4) + HOOH; and (iv) H(2)O(4) rearranges to the cyclic dimer (HO(2))(2), which in turn forms HOOH plus (1)O(2) or (3)O(2). Pathway II differs in that step ii is replaced with the reaction between HOOOH and (1)O(2), leading to the formation of HOO-HOOO. This then proceeds to similar products. For a system with (18)O H(2)O, Pathway I leads to a 2.2:1 ratio of (16)O:(18)O 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 (1)O(2) or one, depending on whether the product of step iv is (1)O(2) or (3)O(2), in good agreement with the experimental result of 2.0. In addition to the Ab-induced reactions, the hydrogen polyoxides (H(2)O(3) and H(2)O(4)) formed in these mechanisms and their decomposition product polyoxide radicals (HO(2), HO(3)) may play a role in combustion, explosions, atmospheric chemistry, and the radiation chemistry in aqueous systems.