Structural Effects on the pH-Dependent Redox Properties of Organic Nitroxyls: Pourbaix Diagrams for TEMPO, ABNO, and Three TEMPO Analogs

Electrochemical studies of the reduction and oxidation reactions of five different organic nitroxyls have been performed across a wide pH range (0-13). The resulting Pourbaix diagrams illustrate structural effects on their various redox potentials and on the pK(a) values of the corresponding hydroxylamine and hydroxylammonium ions. Evidence is also given for the reversible formation of a hydroxylamine N-oxide when nitroxyls are oxidized in alkaline media. Structural effects on the thermodynamics of this reaction are assessed.