Electron transfer between azide and chlorine dioxide: The effect of solvent barrier nonadditivity

Article abstract of DOI:10.1021/ja00062a030

The reaction of chlorine dioxide with excess azide in aqueous media proceeds with complex kinetics and produces N₂, N₂O, NO₃^-, Cl^-, and ClO₂^-. In the presence of the spin trap PBN, the reaction is much simpler, and the rate law is -d [ClO₂]/dt = k₁ [ClO₂] [N₃^-] [PBN]/([PBN] + [ClO₂^-]k_-1/k₂), with k₁ = 809 M^-1 s^-1 and k_-1/k₂ = 19.0 at 25 °C. The inferred mechanism implies that k₁ is the rate constant of electron transfer between ClO₂ and N₃^-, k_-1 is the reverse rate constant (N₃ with ClO₂^-), and k₂ is the rate constant for reaction of N₃ with PBN. A dramatically lower value for k₁ of 0.62 M^-1 s^-1 is calculated from the Marcus cross relationship and literature values for the self-exchange rates. The discrepancy is attributed to systematic errors in the literature self-exchange rates that were derived by applying the Marcus cross relationship to reactions of coordination complexes with N₃^- and ClO₂. Such errors develop whenever this method is applied to reactions between species of widely differing size. Correcting for this effect leads to a calculated value of 56 M^-1 s^-1 for k₁, which is in much improved agreement with the observed value. Similar corrections lead to greatly improved correlations for the self-exchange reaction of NO₂ with NO₂^- and the electrontransfer reaction of ClO₂ with NO₂^-.