Peroxone chemistry: Formation of H₂O₃ and ring-(HO₂)(HO₃) from O₃/H₂O₂

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 H₂O₂ from ¹O₂ plus H₂O was explained in terms of the formation of the H₂O₃ species that in the antibody react5 with a second H₂O₃ to form H₂O₂. There have been few reports of the chemistry for forming H₂O₃, 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 H₂O₃. 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 H₂O₂ and O₃ to form a [(HO₂)(HO₃)] hydrogen-bonded complex [head-to-tail seven-membered ring (7r)]. In this complex, the barrier for forming H₂O₃ plus ³O₂ is only 4.8 kcal/mol, which should be observable by means of thermal processes (not yet reported). Irradiation of the [(HO₂)(HO₃)-Tr)-7r] complex should break the HO-OO bond of the HO₃ moiety, eliminating ³O₂ and leading to [(HO₂)(HO)]. This [(HO₂)(HO)] confined in the matrix cage is expected to rearrange to also form H₂O₃ (observed experimentally). We show that these two processes can be distinguished isotopically. These results (including the predicted vibrational frequencies) suggest strategies for synthesizing H₂O₃ and characterizing its chemistry. We suggest that the [(HO₂)(HO₃)-7r] complex and H₂O₃ are involved in biological, atmospheric, and environmental oxidative processes.