Application of the Wedge Scheme to Explain Quinone-Phenol Electrochemical Systems

Abstract

Schemes involving chemical reactions, usually hydrogen bonding or proton transfer, and electron transfers have been used for decades to describe electrochemical process important to biology,1 medicine,2,3 and fundamental chemistry.4 These schemes often take advantage of the fact that many of them are thermodynamic cycles, of “square schemes,” which allow us to determine the free energy of reaction of any one step if we know the values for the other steps. The fact that such systems appear so often in organic electrochemistry is due to the twin reasons that (1) adding or subtracting electrons from organic molecules often perturbs the molecule enough to cause another reaction, thus giving rise to such schemes, and (2) voltammetry is a convenient means of determining reaction thermodynamics. Recently, two of these squares have been combined into a 3-dimensional “wedge” scheme in order to explain the electrochemistry of an electroactive urea.5,6 Here we report the use of this wedge scheme to explain the voltammetry of the quinone-phenol system. This system is very important because it is present in the electron transport chains present in both chloroplasts and mitochondria and is therefore an important area of study for both pharmaceuticals and energy transduction. 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. 1. Thayer, W. S. and Hinkle, P. C. The Journal of Biological Chemistry (1975) vol 250, pp 5330-5335. 2. Rao, Gopalakrishna M.; Lown, J. William; and Plambeck, James A. Journal of the Electrochemical Society (1978) vol 175, iss 4, pp 540-543. 3. Andres, Theresa; Eckmann, Lars; and Smith, Diane K. Electrochimica Acta vol 92, pp 257-268. 4. Bonin, Julien; Costentin, Cyrille; Louault, Cyril; Robert, Marc; and Saveant, Jean-Michel. Journal of the American Chemical Society (2011) vol 133, iss 17, pp 6668-6674. 5. Clare, Laurie A.; Pham, An T.; Magdaleno, Francine; Acosta, Jaqueline; Woods, Jessica E.; Cooksy, Andrew L.; and Smith, Diane K. Journal of the American Chemical Society (2013) vol 135, iss 50, pp 18930-18941. 6. Staley, Patrick A.; Newell, Christina; Pullman, David; and Smith, Diane K. Analytical Chemistry (2014) vol 86, iss 21, pp 10917-10924. 7. Ge, Yu; Lilienthal, Ronald R.; and Smith, Diane K. Journal of the American Chemical Society (1996) vol 118, iss 16, pp 3976-3977.