Reinvestigation of a former concerted proton-electron transfer (CPET), the reduction of a hydrogen-bonded complex between a proton donor and the anion radical of 3,5-di-tert-butyl-1,2-benzoquinone.

In 2001, Lehmann and Evans (J. Phys. Chem. B, 2001, 105, 8877-8884) reported that the electrochemical reduction of a hydrogen-bonded complex between a proton donor and the anion radical of 3,5-di-tert-butyl-1,2-benzoquinone in acetonitrile proceeded by a concerted proton-electron transfer (CPET) reaction in which electron transfer from the electrode and proton transfer from proton donor to the quinone moiety occurred concertedly. Support for this conclusion was based upon ruling out both of the competing two-step processes, electron transfer followed by proton transfer (EP) and proton transfer followed by electron transfer (PE). In the course of studies of related compounds it was decided to reinvestigate the reduction of 3,5-di-tert-butyl-1,2-benzoquinone. It was discovered that the earlier conclusion that a CPET reaction was occurring was tenable only for the particular electrolyte that was used, tetrabutylammonium hexafluorophosphate and for lower concentrations of the quinone. Even the small change of carrying out the reduction of the quinone in the presence of water with tetramethylammonium hexafluorophosphate as electrolyte, produced voltammograms with clear signatures that the process was EP rather than CPET. Even more dramatic effects were seen with cesium, potassium or sodium ions in the electrolyte. A general reaction scheme to explain results with all electrolytes will be presented.