Electrochemical oxidation and reduction of nitroxides : A cyclic voltammetric and simulation study

2231-2237The oxidation and reduction of the nitroxides namely, TEMPO (2,2,6,6-tetramethyl piperidine-N-oxyl). TEMPOL (4-hydroxy 2,2,6,6-tetramethyl piperidine-N-oxyl). TEMPO-NH2 or Tempamine (4-amino 2,2,6,6-tetramethyl piperidine-N-oxyl) 3 -carbamoyl proxyl and 4- methyl sulphonyloxy-TEMPO have been followed by cyclic voltammetry in aqueous medium at pH 6.8. The E1/2 values for the oxidation of these nitroxides to oxoammonium cation and then back to parent compound were found to be in the range of 0.85 to 1.0 V vs N HE. The diffusion coefficients (Dox) are of the order of 105 cm2/s and are dependent on the size of the molecule. The process is reversible, with a small amount undergoing chemical reaction after charge transfer. The rate constants for the forward chemical reaction after oxidation (kf) are of the order or 103 S-1. It has been observed that kf is the highest for NH2-TEMPO where the reversibility of the reaction is the lowest. In the negative potentials, where the reduction of nitroxides to hydroxylamines takes place irreversible waves have been observed. The reduction process or the above nitroxides when studied in methanol and acetonitrile also shows irreversibility. Since the reduction process of these nitroxides shows poor reversibility, methods like Condecon (a computer software) has been used to calculate E1/2 values and other parameters like (Do) and (αna) ' This method has been found very effective for obtaining reduction potentials of nitroxides in aqueous solutions