THE EC-CATALYTIC MECHANISM AT THE ROTATING-DISK ELECTRODE .2. COMPARISON OF APPROXIMATE THEORIES FOR THE 2ND-ORDER CASE AND APPLICATION TO THE REACTION OF BIPYRIDINIUM CATION RADICALS WITH DIOXYGEN IN NONAQUEOUS SOLUTIONS

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 k1 in acetonitrile ranged from 5 × 103 M-1 s-1 to 2 × 108 M-1 s-1. The dependence of log k1 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. © 1990.