Peroxone chemistry: Formation of H_2O_3 and ring-(HO_2)(HO_3) from O_3/H_2O_2

Abstract

The recent observation [Wentworth, P., Jones, L. H., Wentworth, A. D., Zhu, X. Y., Larsen, N. A., Wilson, I. A., Xu, X., Goddard, W. A., Janda, K. D., Eschenmoser, A. & Lerner, R. A. (2001) Science 293, 1806–1811] that antibodies form H_2O_2 from ^1O_2 plus H_2O was explained in terms of the formation of the H_2O_3 species that in the antibody reacts with a second H_2O_3 to form H_2O_2. There have been few reports of the chemistry for forming H_2O_3, but recently Engdahl and Nelander [Engdahl, A. & Nelander, B. (2002) Science 295, 482–483] reported that photolysis of the ozone–hydrogen peroxide complex in argon matrices leads to significant concentrations of H_2O_3. We report here the chemical mechanism for this process, determined by using first-principles quantum mechanics. We show that in an argon matrix it is favorable (3.5 kcal/mol barrier) for H_2O_2 and O_3 to form a [(HO_2)(HO_3)] hydrogen-bonded complex [head-to-tail seven-membered ring (7r)]. In this complex, the barrier for forming H_2O_3 plus ^3O_2 is only 4.8 kcal/mol, which should be observable by means of thermal processes (not yet reported). Irradiation of the [(HO_2)(HO_3)-7r] complex should break the HO–OO bond of the HO_3 moiety, eliminating ^3O_2 and leading to [(HO_2)(HO)]. This [(HO_2)(HO)] confined in the matrix cage is expected to rearrange to also form H_2O_3 (observed experimentally). We show that these two processes can be distinguished isotopically. These results (including the predicted vibrational frequencies) suggest strategies for synthesizing H_2O_3 and characterizing its chemistry. We suggest that the [(HO_2)(HO_3)-7r] complex and H_2O_3 are involved in biological, atmospheric, and environmental oxidative processes.

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