Mechanistic - Kinetic scheme of oxidation/reduction of TEMPO involving hydrogen bonded dimer. RDE probe for availability of protons in reaction environment

Mechanistic - kinetic studies on the electrochemical oxidation/reduction process of radical TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) under ionic strength (0.1 M, 1.0 M) and pH (0, 7) of aqueous perchlorate electrolyte (NaClO4-HClO4) have been undertaken. Analytical and/or digital simulation methods for voltammetry at stationary (CV) and rotating electrode (RDE) have allowed one to determine numerical values of twelve parameters characterizing two electrode reactions (oxidation and reduction of the radical) and three chemical reactions (protonation, disproportionation, dimerization involving the radical and/or electrogenerated species). A potential window of the measurements was 0.6 V and it corresponded to that where the oxidation wave of TEMPO in neutral aqueous solution is situated. To account for the observed pH effect, the hydrogen bonded dimer resulting from the radical reactant and the protonation product of its reduction has been postulated to form in solution near the electrode surface. The RDE voltammetric discernables of the TEMPO process (i.e., absolute RDE wave current, zero RDE current potential, oxidation and reduction limiting RDE currents) can be considered good candidates for a use to follow acidity of complex reactive media.