Electrochemical Concerted Proton and Electron Transfers. Further Insights in the Reduction Mechanism of Superoxide Ion in the Presence of Water and Other Weak Acids

Deciphering of the role of water (or other weak acids) in the reduction of superoxide is based on three observations: (i) very large peak potential shifts (and hence a large increase of the apparent standard rate constant) associated with the reaction O2•----H2O + e- → -O2H---OH- upon addition of water (or methanol, or 2-propanol) to the acetonitrile solution; (ii) increase of the hydrogen/deuterium isotope effect with water concentration; (iii) the fact that the positive shift of the peak potential (or equivalently the increase of the standard reduction rate constant) is larger with methanol than with water. Mere invocation of specific solvation by water (or the other weak acids) of the reduction products cannot explain these facts. They instead reveal that short hydrogen-bonded water chains, involving approximately three water molecules, are involved in the concerted proton−electron transfer process, thus providing a preliminary picture of the mechanisms operating in pure water or other H-bonding and H-bonded media.