Structural Properties of 4-Substituted Phenols Capable of Proton-Coupled Electron Transfer to Superoxide

The structural properties of 4-substituted phenols capable of proton-coupled electron transfer (PCET) to superoxide (O2 ) were investigated on the basis of electrochemical and electron spin resonance (ESR) spectral measurements and supported by density functional theory calculations. Although the proton and electron donating abilities of phenols are considered to be important factors in PCET reactions, the major mechanism for the reactions between 4-substitued monophenols and O2 •− is the proton-transfer pathway that is independent of the electronic substituent effect. Conversely, the presence of 4-aminophenol in addition to p-hydroquinone distinctly affects the reversible O2/O2 •− redox pair; the associated PCET reaction occurs from the 4-aminophenol to O2 •− to give a stable intermediate radical in the redox system. The generation of p-benzoquinone imine radical was inferred from the results obtained from ESR spectral measurements. B3LYP/PCM/6-31+G(d,p) calculations indicate that the O2 -scavenging reaction involves a primary proton transfer from 4-aminophenol to O2 •− to give HO2 , followed by a one-step concerted PCET reaction to yield p-benzoquinone imine radical and H2O2. The structural properties of 4-aminophenol are important in the O2 -scavenging PCET reaction; 4-aninophenol is characterized by a  electronic redox system involving two protons and two electrons, similar to the quinone–hydroquinone redox system.