SCANNING ELECTROCHEMICAL MICROSCOPY .16. STUDY OF 2ND-ORDER HOMOGENEOUS CHEMICAL-REACTIONS VIA THE FEEDBACK AND GENERATION COLLECTION MODES

The application of the scanning electrochemical microscope feedback and generation/collection (G/C) modes in the measurement of second-order homogeneous reactions of electrogenerated species (i.e., the E(r)C2i mechanism) is considered, with particular emphasis on dimerization. Two G/C modes are assessed: (i) tip generation/substrate collection (TG/SC) and (ii) substrate generation/tip collection (SG/TC). The TG/SC mode is shown to be preferable for kinetic studies in terms of the higher collection efficiencies attainable under steady-state conditions. A numerical treatment of the feedback and TG/SC problem, which relates the tip (feedback) and substrate (collection) currents to the tip-substrate separation, the electrode radii, and dimerization rate constant, is developed, and an extensive set of calculated steady-state feedback and collection characteristics is presented that allow construction of appropriate working curves. The theoretical results suggest that fast dimerization rate constants, up to 4 x 10(8) M-1 s-1 (defined in terms of the rate of loss of the monomer), should be accessible to measurement in the steady-state TG/SC mode. The application of the proposed methodology is demonstrated through studies of the reductive coupling of both dimethyl fumarate (DF) and fumaronitrile (FN) in N,N-dimethylformamide. Good agreement between theory and experiment is displayed over a wide range of concentrations, yielding mean values for the dimerization rate constants of 170 M-1 s-1 (DF) and 2.0 x 10(5) M-1 s-1 (FN).