Surface-Enhanced Raman Spectroscopy as a Probe of Electroorganic Reaction Pathways. 2. Ring-Coupling Mechanisms during Aniline Oxidation

Article abstract of DOI:10.1021/j100346a075

The adsorbed ring-coupling products formed during aniline electrooxidation at gold-aqueous interfaces have been identified by means of real-time surface-enhanced Raman-spectroscopy (SERS) in conjunction with linear sweep voltammetry.In the absence of solution species, the electrooxidation of irreversibly adsorbed aniline yields predominantly adsorbed benzidine.In the presence of solution aniline, however, the "head-to-tail" dimer N-phenyl-1,4-phenylenediamine (PPDA) as well as benzidine is formed at the interface, the proportion of the former adsorbate increasing toward higher pH and larger aniline concentrations.A distinction between these interfacial reaction products can readily be made from the characteristic and intense SER spectra of the two-electron oxidation products formed from PPDA and benzidine.Markedly different ring-coupling product distributions were formed in solution (i.e., within the diffusion layer) as deduced voltammetrically, PPDA predominating except in strong acidic media.Possible reasons for these observed differences are discussed.In alkaline media, azobenzene is observed to be the major ring-coupling product in both adsorbed and solution-phase environments.