School of Chemistry, Monash University, Clayton, Victoria 3800, Australia.
J Phys Chem A. 2010 Sep 23;114(37):10122-34. doi: 10.1021/jp105626z.
Large-amplitude ac voltammograms contain a wealth of kinetic information concerning electrode processes and can provide unique mechanistic insights compared to other techniques. This paper describes the effects homogeneous chemical processes have on ac voltammetry in general and provides experimental examples using two well-known chemical systems: one simple and one complex. Oxidation of Cp*Fe(CO)(2) (Cp* = η(5)-pentamethylcyclopentadienyl) in noncoordinating media is a reversible one-electron process; in the presence of nucleophiles, however, the resulting ligand-induced disproportionation changes the process to a multiple step regeneration. The chemical kinetic parameters of the regeneration mechanism were discerned via analysis of the third and higher harmonics of Fourier-transformed ac voltammetry data. Comparison of experimental data to digital simulations provides clear evidence that the reaction proceeds via a rapid pre-equilibrium between the electrogenerated monocation and the coordinating ligand; simultaneous fitting of the first nine harmonics indicates that k(f) = 7500 M(-1) s(-1) and k(r) = 100 s(-1), and that the unimolecular decomposition of the corresponding intermediate occurs with a rate constant of 2.2 s(-1). The rapid cis(+) → trans(+) isomerization of the electrogenerated cis-W(CO)(2)(dpe)(2), where dpe = 1,2-diphenylphosphinoethane, was examined to illustrate the effects of a simpler EC mechanism on the higher harmonics; a rate constant of 280 s(-1) was determined. These results not only shed new light on the chemistry of these systems, but provide a clear demonstration that the higher harmonics of ac voltammetry provide mechanistic insights into coupled homogeneous processes far more detailed than those that are readily accessible with dc techniques.
大振幅交流伏安图包含有关电极过程的丰富动力学信息，与其他技术相比，可以提供独特的机理见解。本文描述了均相化学过程对交流伏安法的一般影响，并提供了两个著名化学系统的实验实例：一个简单，一个复杂。Cp*Fe(CO)(2)（Cp*=η(5)-戊甲基环戊二烯基）在非配位介质中的氧化是一个可逆的单电子过程；然而，在亲核试剂存在下，由此产生的配体诱导歧化反应将过程改变为多步再生。通过分析傅里叶变换交流伏安法数据的第三和更高次谐波，辨别了再生机制的化学动力学参数。实验数据与数字模拟的比较为反应通过电生成的单阳离子和配位配体之间的快速预平衡进行提供了明确的证据；同时拟合前九个谐波表明，k(f)=7500 M(-1) s(-1)和 k(r)=100 s(-1)，并且相应中间物的单分子分解以 2.2 s(-1)的速率常数发生。电生成的顺式-W(CO)(2)(dpe)(2)（其中 dpe=1,2-二苯基膦乙烷）的快速顺式（+）→反式（+）异构化被研究以说明更简单的 EC 机制对更高次谐波的影响；确定了 280 s(-1)的速率常数。这些结果不仅揭示了这些系统的化学性质，而且还清楚地表明，交流伏安法的更高次谐波提供了对耦合均相过程的机理见解，远比直流技术更容易获得的那些详细。
