Outer-Sphere Electron-Transfer Reactions Involving the Chlorite/Chlorine Dioxide Couple. Activation Barriers for Bent Triatomic Species

Article abstract of DOI:10.1021/ja00322a018

The kinetics of several redox reactions involving the ClO2/ClO2^- couple have been determined in aqueous solution by stopped-flow spectrophotometry.ClO2 is reduced by ²⁺ to produce ClO2^- and ³⁺ with simple bimolecular kinetics (k=2.1E7 M^-1 s^-1 at 25 deg C, μ=0.1 M (NaCF3SO3)).ClO2^- is oxidized by IrCl6^2- to produce ClO2 and IrCl6^3-; the rate law is -d ln 2->/dt = k₁/(1++>/K_a), with k₁=1.06E4 M^-1 s^-1 and K_a=1.6E-2 M, the acid dissociation constant of HClO2.For the reaction of ClO2^- with IrBr6^2- k₁ is 1.86E4 M^-1 s^-1.Application of the Marcus-Hush cross relationship to these outer-sphere electron-transfer reactions leads to a self-consistent self-exchange rate constant of 1.6E2 M^-1 s^-1 for the ClO2/ClO2^- couple.An explicit equation for the classical contributions of molecular vibrations to the activation free energy of self-exchange reactions of bent triatomic species has been derived.Calculations of these barriers show that both bending and stretching are important in the activation process.With this equation the activation barriers for the ClO2/ClO2^-, NO2/NO2^-, and SO2/SO2^- redox couples have been rationalized.Nuclear tunneling introduces a correction to the classical rate constant by a factor of 79 for the NO2/NO2^- couple.