Univ Paris Diderot, Sorbonne Paris Cité, Laboratoire d'Electrochimie Moléculaire, Unité Mixte de Recherche Univ-CNRS No 7591, Bâtiment Lavoisier, 15 rue Jean de Baïf, 75205 Paris Cedex 13, France.
J Am Chem Soc. 2011 Nov 30;133(47):19160-7. doi: 10.1021/ja206561n. Epub 2011 Nov 9.
The concerted proton-electron transfer (CPET) oxidation of phenol with water (in water) and hydrogen phosphate as proton acceptors provides a good example for testing the consistency of the electrochemical and homogeneous approaches to a reaction, the comprehension of which raises more mechanistic and kinetic challenges than that of a simple outer-sphere electron transfer. Comparison of the intrinsic kinetic characteristics (obtained at zero driving force of the CPET reaction) shows that consistency is indeed observed after a careful identification and quantitation of side factors (electrical work terms, image force effects). Water (in water) appears as a better intrinsic proton acceptor than hydrogen phosphate in both cases in terms of reorganization energy and pre-exponential factor, corroborating the mechanism by which electron transfer is concerted with Grotthus-type proton translocation in water. Detailed compared analysis of the approaches also revealed that modest but significant electric field effects may be at work in the electrochemical case. Comparison with phenoxide ion oxidation, taken as a reference outer-sphere electron transfer, points to a CPET precursor complex that possesses a precise spatial structure allowing the formation of one or several H-bonds as required by the occurrence of the CPET reaction, thus decreasing considerably the number of efficient collisions compared with those undergone by structureless spherical reactants.
协同质子-电子转移 (CPET) 氧化苯酚与水 (在水中) 和磷酸氢作为质子受体，为测试电化学和均相方法对反应的一致性提供了一个很好的例子，对该反应的理解比简单的外层电子转移提出了更多的机制和动力学挑战。内在动力学特性的比较（在 CPET 反应的零驱动力下获得）表明，在仔细识别和量化了侧因子（电功项、映像力效应）后，确实观察到了一致性。就重组能和指数前因子而言，在这两种情况下，水（在水中）都比磷酸氢作为更好的内在质子受体，这证实了在水中协同电子转移与 Grotthus 型质子迁移的机制。对两种方法的详细比较分析还表明，电化学情况下可能存在适度但显著的电场效应。与作为参考外层电子转移的苯酚氧离子氧化的比较表明，CPET 前体复合物具有精确的空间结构，允许形成一个或多个氢键，这是 CPET 反应发生所必需的，因此与无结构的球形反应物相比，大大减少了有效的碰撞次数。
