Electrode reactions of nitroxide radicals. IX. Anodic oxidations of 4-hydroxyimino-2,2,6,6-tetramethylpiperidine-1-oxyl and 4-[(aminocarbonyl)-hydrazone]-2,2,6,6-tetramethylpiperidine-1-oxyl in water solutions

Anodic reactions of two stable nitroxide radicals, 4-hydroxyimino-2,2,6,6-tetramethylpiperidine-1-oxyl (1) and 4-[(aminocarbonyl)-hydrazone]-2.2,6,6-tetramethylpiperidine-1-oxyl (2) were studied in aqueous solutions over the pH range 5.33–8.04 using the cyclic voltammetry technique. They show one-electron oxidation processes of reversible and quasi-reversible natures, depending on scan rate conditions at pH 5.33. The diffusion coefficients, the observed standard rate constants (ks), and the oxidation potentials for the redox reaction nitroxyl radical ⇌ oxoammonium salt have been determined. A one-electron transfer process, followed by an irreversible chemical reaction, is observed at pH ≥ 5.71, and the rates of this reaction at pH 5.71 and 6.71 are 0.11 and 0.080 s−1, respectively. Controlled-potential electrolysis at 0.8 V (vs. SCE) confirms a one-electron electrode reaction.