Scanning Electrochemical Microscopy Determination of Heterogeneous Electron Transfer and Homogeneous Association and Comproportionation Rate Constants of Anthrarufin

Abstract

AbstractWe report the use of scanning electrochemical microscopy (SECM) in determining the heterogeneous electron transfer and homogeneous association and comproportionation kinetics of anthrarufin (HQ) in acetonitrile at a Pt tip UME (radius 12.5 µm). HQ undergoes two consecutive one‐electron reductions with comproportionation occurring between HQ2−, the product of the second reduction, and bulk HQ to produce HQ− and association occurring between HQ− and bulk HQ to produce an electroinactive species. Standard rate constants, k10=5.1±0.4 cm/s, for the first reduction, and a comparatively smaller rate constant, k20=0.37±0.03 cm/s, for the second reduction, were determined by tip voltammetry over a large Pt substrate. An association rate constant, kassoc=2.9×102 M−1 s−1, was determined using positive feedback approach curves over co‐axially aligned tip and substrate UMEs of the same radius. A comproportionation rate constant, kcomp=7.1×101 M−1 s−1, was determined in the presence of the association reaction by tip pulse chronoamperometry at the same co‐axially aligned tip and substrate UMEs. Diffusion coefficients of the HQ species and standard potentials for the reductions were also determined. Experimental results were compared with 2D transient axisymmetric simulations.