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

Abstract

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 (^1O_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_2O catalyzes the reaction of ^1O_2 with a second water to form HOOOH; (ii) two HOOOH form a dimer, which rearranges to form the HOO-HOOO + H_2O complex; (iii) HOO-HOOO rearranges to HOOH-OOO, which subsequently reacts with H_2O to form H_2O_4 + HOOH; and (iv) H_2O_4 rearranges to the cyclic dimer (HO_2)_2, which in turn forms HOOH plus ^1O_2 or ^3O_2. Pathway II differs in that step ii is replaced with the reaction between HOOOH and ^1O_2, leading to the formation of HOO-HOOO. This then proceeds to similar products. For a system with ^(18)O H_2O, 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 ^1O_2 or one, depending on whether the product of step iv is ^1O_2 or ^3O_2, in good agreement with the experimental result of 2.0. In addition to the Ab-induced reactions, the hydrogen polyoxides (H_2O_3 and H_2O_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.

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