The EC-catalytic mechanism at the rotating disk electrode: Part II. Comparison of approximate theories for the second-order case and application to the reaction of bipyridinium cation radicals with dioxygen in non-aqueous solutions

Abstract

Orthogonal collocation was used to obtain an efficient iterative solution to the problem of the second-order EC-catalytic mechanism at the rotating disk electrode (RDE). Application of the spline collocation technique provided accurate results for large values of the kinetic parameter or the substrate/catalyst concentration ratio. The numerical solution was in good agreement with approximate analytical solutions available for certain limiting cases. The second-order EC-catalytic mechanism at the RDE was used to measure the rate of reaction between bipyridinium cation radicals and dioxygen in non-aqueous solutions. Sixteen diquaternized derivatives of 2,2′- and 4,4′-bipyridine were studied including the commercially available herbicides diquot and paraquat (methyl viologen). Values obtained for the rate constant k 1 in acetonitrile ranged from 5 × 10 3 M −1 s −1 to 2 × 10 8 M −1 s −1. The dependence of log k 1 on the standard potentials of the reactants conformed to the Marcus equation for an activation-controlled reaction. The result obtained for the Gibbs energy of activation, 13.1 kJ/mol, was in good agreement with that calculated using the standard rate constant for the heterogeneous reduction of dioxygen. The rate constants in acetonitrile also showed a strong correlation with published data on the herbicidal activity of selected bipyridinium compounds in mustard and tomato.