Heterogeneous Proton Transfer As an Explanation for the Unusually Thick Cyclic Voltammograms and Unusually Small Second Redox Wave of Para-Quinones in Aprotic Solvents

氧化还原 化学 二聚体 循环伏安法 光化学 单体 玻璃碳 自由基离子 离子 电化学 电极 无机化学 物理化学 有机化学 聚合物
作者
Patrick Andrew Staley,Diane K. Smith
出处
期刊:Meeting abstracts 卷期号:MA2014-01 (22): 990-990
标识
DOI:10.1149/ma2014-01/22/990
摘要

Quinones are well-known to undergo two reversible reductions at glassy carbon electrodes in aprotic solvents, first to a radical anion and then to a dianion, eq 1. Despite 50+ years of voltammetric investigation of this chemistry, two unusual features of quinone voltammetry—that the second redox wave is smaller than the first, and that the area between the two waves is thicker than CV simulations say it should be—was typically ignored until Gupta and Linschitz made note of the difference in height of the redox waves in 1997.1 The current explanation for the unequal wave heights, advanced by Macías-Ruvalcaba and Evans in 2010,2 is that some of the radical anion is trapped in the form of a pi dimer in solution. Macias-Ruvalcaba and Evans hypothesize that this dimer can be reduced at the same potential as the monomer radical anion, but only by 1 e- per dimer, resulting in smaller current than expected. There currently is no explanation in the literature for the thickness between the two waves. In this study, concentration dependence of the CVs of differently substituted quinones showed that radical dimerization cannot be a general explanation for the small second redox wave. In some cases the thickness between the waves appears as its own quasi-reversible wave. Addition of phenols to a quinone solution produces a quasi-reversible wave at the same potential due to hydrogen bonding to the radical anion. The McCreery lab showed that oxide content of the surface of glassy carbon can change its chemistry through the presence of functional groups such as phenols.3 In low concentration duroquinone studies, different surface treatments show that with higher oxide content a new wave grows in that is at the same potential as the wave seen in a 1:1 addition of naphthol to the quinone redox system. This indicates that the odd thickness in quinone CV’s may be due to hydrogen bonding of the quinones to phenolic groups on the glassy carbon surface followed by proton transfer. REFERENCES 1. Gupta, J. ;Linschitz, H. Hydrogen Bonding and Protonation Effects in Electrochemistry of Quinones in Aprotic Solvents. J. Am. Chem. Soc., 1997, 119(27), 6384-6391. 2. Macías-Ruvalcaba, N.; Evans, D. Association Reactions of the Anion Radicals of Some Hydroxyquinones: Evidence for the Formation of π and σ–Dimers As Well As a Neutral-Anion Radical Complex. J. Phys. Chem. C, 2010, 114(2), 1285-1292. 3. Kuo, T.; McCreery, R.L. Surface Chemistry and Electron-Transfer Kinetics of Hydrogen-Modified Glassy Carbon Electrodes. Anal. Chem. 1999, 71, 1553-1560.

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