Spatially Resolved Spectroelectrochemical Examination of the Oxidation of Dopamine by Chlorpromazine Cation Radical

Article abstract of DOI:10.1021/j100372a047

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 x 10⁵ M^-1s^-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.