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Detection of the Short-Lived Cation Radical Intermediate in the Electrochemical Oxidation of N,N‑Dimethylaniline by Scanning Electrochemical Microscopy

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journal contribution
posted on 31.12.2014, 00:00 by Fahe Cao, Jiyeon Kim, Allen J. Bard
The short-lived intermediate N,N-dimethyl­aniline (DMA) cation radical, DMA•+, was detected during the oxidation of DMA in MeCN with 0.1 M tetra-n-butyl­ammonium hexafluoro­phosphate. The detection was accomplished at steady state by scanning electrochemical microscopy (SECM) with ultramicroelectrodes using the tip generation/​substrate collection mode. Cyclic voltammetry (CV) with a 2 mm Pt electrode indicates that DMA oxidation in acetonitrile is followed by a dimerization and two electrochemical reactions, which is consistent with previous results. The DMA•+ intermediate is detected by SECM, where the DMA•+ generated at the ca. 500 nm radius Pt tip is collected on a 5 μm radius Pt substrate when the gap between the tip and the substrate is a few hundred nanometers. Almost all of the DMA•+ is reduced at the substrate when the gap is 200 nm or less, yielding a dimerization rate constant of 2.5 × 108 M–1·s–1 based on a simulation. This is roughly 3 orders of magnitude larger than the value estimated by fast-scan CV. We attribute this discrepancy to the effects of double-layer capacitance charging and adsorbed species in the high scan rate CV.