SPATIALLY RESOLVED SPECTROELECTROCHEMICAL EXAMINATION OF THE OXIDATION OF DOPAMINE BY CHLORPROMAZINE CATION RADICAL

Spatially resolved absorption measurements of electrogenerated chlorpromazine cation radical (CPZ•+) were used to monitor the kinetics of the homogeneous oxidation of dopamine (DA) in the solution near planar and cylindrical electrodes. Complete concentration vs distance profiles for CPZ•+ were obtained for both planar and convergent diffusion and were used to determine the reaction mechanism and rate constants. The oxidation of DA by CPZ•+ to dopamine orthoquinone involves successive one-electron transfers, and the results are inconsistent with disproportionation of the DA semiquinone. The observed kinetics and rate-limiting step were dependent on the position within the diffusion layer. Near the electrode surface the rate law is first order in CPZ•+ and DA, and a second-order rate constant of 2.1 × 105 M-1 s-1 was obtained. Further away from the electrode the reaction is second order in CPZ•+ due to the involvement of the second step, and several kinetic parameters for the second electron transfer were obtained. A change in diffusion geometry which occurs at a microwire electrode further perturbs local concentration distributions, and the reaction reaches equilibrium under certain conditions. In all cases, the observed rate laws were consistent with a general expression for the stepwise electron transfer. This report represents the first application of spatially resolved spectroelectrochemistry to a kinetic system and provides unprecedented detail about the CPZ•+/DA reaction. © 1990 American Chemical Society.