ELECTRON-TRANSFER TO OZONE - OUTER-SPHERE REACTIVITIES OF THE OZONE OZONIDE AND RELATED NONMETAL REDOX COUPLES

For the net reaction O3 + 2lrCl63− + 2H+ = O2 + 2IrCl62− + H2O, d[IrCl62−]/dt = 2k[O3][IrCl63−], with k = 1.7 × 104 M−1 s−1 (25 °C), ΔHǂ = 10.3 kcal mol−1, and ΔSǂ = −4.5 eu. The initial step appears to be an outer-sphere electron transfer from IrCl63− to O3, followed by rapid protonation of O3− and oxidation of a second IrCl63− by HO3 or OH. From a reduction potential of 1.02 V for the O3/O3− couple, its self-exchange rate constant is calculated to be 4 M−1 s−1; an estimate of 90 M−1 s−1 results from a calculation of the primary bond and solvation shell rearrangement energies. Similar approximate agreement is obtained between the self-exchange constants calculated by the relative and absolute Marcus-Hush theories for the O2/O2−, ClO2/ClO2−, NO2/NO2−, and N3/N3− couples so long as cross-reactions with inert metal complexes are employed. In contrast, the cross-reactions NO2/O2−, O3/ClO2−, O3/NO2−, ClO2/NO2−, and O2/SO2− are orders of magnitude faster than predicted from these self-exchange constants. These higher reactivities appear attributable to enhanced orbital overlap between two non-metal reactants in the transition state. Oxidative additions of O3 to U4+(aq) and Fe2+(aq) to yield monodentate overlapozonide intermediates, from which O2 or O3− is lost, appear likely. Thus, a broad range of redox mechanisms, from weak-overlap, outer-sphere electron transfer through strong orbital overlap in both transition state and product(s), is exhibited by ozone. © 1990, American Chemical Society. All rights reserved.