Two-electron transfer for Tl(aq)(3+)/Tl(aq)(+) revisited. Common virtual [Tl(II)-Tl(II)](4+) intermediate for homogeneous (superexchange) and electrode (sequential) mechanisms.
Two-electron transfer for Tl(aq)(3+)/Tl(aq)(+) revisited. Common virtual [Tl(II)-Tl(II)](4+) intermediate for homogeneous (superexchange) and electrode (sequential) mechanisms.
复制标题
重新审视 Tl(aq)(3 )/Tl(aq)( ) 的双电子转移。
作者：
D. Khoshtariya;T. Dolidze;L. D. Zusman;G. Lindbergh;J. Glaser
在一个共同的概念基础上考虑了均匀和电化学双电子转移的Tl（aq）（3+）/Tl（aq）（+）对。对于Tl（aq）（3+）到Tl（aq）（+）的2当量电化学还原，检测到具有形式还原电位E（1）= 1.04 +/-0.10V（相对于正常氢电极）的中间态，不同于Tl（3+）/Tl（2+）电对的确立值0.33V。对获得的电化学（循环伏安法（CV）和旋转盘电极技术，沿着CV曲线计算机模拟程序）和文献数据的检查表明，检测到的形式电位不能是电极吸附物质的性质，而是位于外亥姆霍兹平面的共价相互作用的双铊中间体[Tl（II）-Tl（II）]（4+）的性质。根据H.陶贝和马库斯-胡什理论（由祖斯曼和贝拉坦，以及科珀和施米克勒扩展）揭示了均匀过程很可能通过超交换内层双电子转移机制，通过一个本质上虚拟的（不可检测的）双铊中间体发生。相比之下，电化学过程通过顺序机制发生，经由Tl（aq）（2+）离子形成的速率决定步骤，紧接着是亚稳态（CV活性）二铊状态的无活化形成。第二个电化学电子转移步骤是快速的，并且仅在所观察到的CV阴极波的峰高（而不是形状）中显示。在所考虑的电位范围内，由于离子转移到电极的竞争过程阻止了空间电子转移，因此无法观察到Tl（aq）（+）氧化为Tl（aq）（3+）的微观逆过程的阳极波。
Homogeneous and electrochemical two-electron transfers within the Tl(aq)(3+)/Tl(aq)(+) couple are considered on a common conceptual basis. For the 2 equiv electrochemical reduction of Tl(aq)(3+) to Tl(aq)(+), the intermediate state with a formal reduction potential, E(1) = 1.04 +/- 0.10 V vs the normal hydrogen electrode, was detected, different from the established value of 0.33 V for a Tl(3+)/Tl(2+) couple. Examination of obtained electrochemical (cyclic voltammetry (CV) and rotating disk electrode techniques, along with the CV-curve computer simulation procedure) and literature data indicate that the detected formal potential cannot be the property of electrode-adsorbed species, but rather of the covalently interacting dithallium intermediate [Tl(II)-Tl(II)](4+) located at the outer Helmholtz plane. The analysis of microscopic mechanisms, based on the recent hypothesis of H. Taube and the Marcus-Hush theory extended by Zusman and Beratan, and Koper and Schmickler, revealed that the homogeneous process most probably takes place through the superexchange inner-sphere two-electron-transfer mechanism, via an essentially virtual (undetectable) dithallium intermediate. In contrast, the electrochemical process occurs through a sequential mechanism, via the rate-determining step of Tl(aq)(2+) ion formation immediately followed by activationless formation of the metastable (CV-active) dithallium state. The second electrochemical electron-transfer step is fast, and shows up only in the peak height (but not in the shape) of the observed CV cathodic wave. The anodic wave for a microscopically reverse process of the oxidation of Tl(aq)(+) to Tl(aq)(3+) cannot be observed within the considered potential range due to the blocking of through-space electron transfer by the competitor process of ion transfer to the electrode.