Clare Laurie A, Rojas-Sligh Lydia E, Maciejewski Sonia M, Kangas Karina, Woods Jessica E, Deiner L Jay, Cooksy Andrew, Smith Diane K
Department of Chemistry and Biochemistry, San Diego State University, San Diego, CA 92182-1030.
J Phys Chem C Nanomater Interfaces. 2010;114(19):8938-8949. doi: 10.1021/jp100079q.
The voltammetry of 2,3,5,6-tetramethyl-p-phenylenediamine, HPD, has been studied and compared to that of its isomer N,N,N'N'-tetramethyl-p-phenylenediamine, MePD. Both undergo two reversible electron transfer processes in acetonitrile that nominally correspond to 1e- oxidation to the radical cations, MePD and HPD, and a second 1e- oxidation at more positive potentials to the quinonediimine dications, MePD and HPD. While the voltammetry of MePD agrees with this simple mechanism, that of HPD does not. The second voltammetric wave is too small. UV/Vis spectroelectrochemical experiments indicate that the second wave does correspond to oxidation of HPD to HPD in solution. The fact that the second wave is not present at all at the lowest concentrations (5 µM), and that it increases at longer times and higher concentrations, indicates that HPD is not the initial solution product of the first oxidation. A number of lines of evidence suggest instead that the initial product is a mixed valent, H-bonded dimer between one HPD in the the full reduced, fully protonated state, HPD, and another in the fully oxidized, fully deprotonated state, PD. A mechanism is proposed in which this dimer is formed on the electrode surface through proton transfer and H-bonding. Once desorbed into solution, it breaks apart via reaction with other HPD's, to give 2 HPD, which is the thermodynamically favored species in solution.
对2,3,5,6-四甲基对苯二胺（HPD）的伏安法进行了研究，并与它的异构体N,N,N',N'-四甲基对苯二胺（MePD）的伏安法进行了比较。两者在乙腈中都经历两个可逆的电子转移过程，名义上对应于1e-氧化为自由基阳离子MePD和HPD，以及在更正的电位下进行第二次1e-氧化为醌二亚胺双阳离子MePD和HPD。虽然MePD的伏安法符合这种简单机制，但HPD的伏安法却并非如此。第二个伏安波太小。紫外/可见光谱电化学实验表明，第二个波确实对应于溶液中HPD氧化为HPD。在最低浓度（5 μM）时根本不存在第二个波，并且它在更长时间和更高浓度下会增加，这一事实表明HPD不是第一次氧化的初始溶液产物。相反，大量证据表明初始产物是一种混合价态的、在一个处于完全还原、完全质子化状态的HPD和另一个处于完全氧化、完全去质子化状态的PD之间形成的氢键二聚体。提出了一种机制，其中这种二聚体通过质子转移和氢键作用在电极表面形成。一旦解吸到溶液中，它会通过与其他HPD反应而分解，生成2个HPD，这是溶液中热力学上更有利的物种。
