Application of the Wedge Scheme to Explain Quinone-Phenol Electrochemical Systems
Cyclic voltammograms of para-quinones usually have just two waves that correspond to reduction to a radical anion and then to the dianion: Q <=> Q- <=> Q2-. With the addition of naphthol, one new reversible wave appears at a potential intermediate of the old two, and one new oxidation peak appears at a potential positive of all the rest. These are very broad at all concentrations of naphthol and at the higher concentrations of naphthol the reduction wave associated with Q <=> Q- also increases. The broadness here could indicate a proton coupled electron transfer (PCET) within a hydrogen bonded complex between Q- and the naphthol which reversibly breaks apart, in which case the proton either ends up on the quinone, which broadens the wave positive, or on the naphthol, which broadens the wave negative. The potentials of the new peaks do not correspond to a pure hydrogen bonded complex because the shift is too large as compared to a related hydrogen bonded complex with 2 hydrogen bonds rather than one.7 The reversibility of the peaks at a range of scan rates argues against a pure PCET mechanism because the reverse wave would be oxidation of a protonated quinone, which should occur at a much shifted potential. Fitted digital simulations, concentration dependence experiments, and scan rate dependence experiments will be discussed.
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