Centre National de la Recherche Scientifique No 7591, Université Paris Diderot, Bâtiment Lavoisier, 75205 Paris Cedex 13, France.
Proc Natl Acad Sci U S A. 2010 Feb 23;107(8):3367-72. doi: 10.1073/pnas.0914693107. Epub 2010 Feb 5.
Three experimental techniques, laser flash photolysis, redox catalysis, and stopped-flow, were used to investigate the variation of the oxidation rate constant of phenol in neat water with the driving force offered by a series of electron acceptors. Taking into account a result previously obtained with a low-driving force electron acceptor thus allowed scanning more than half an electron-volt driving force range. Variation of the rate constant with pH showed the transition between a direct phenol oxidation reaction at low pH, where the rate constant does not vary with pH, and a stepwise reaction involving the prior deprotonation of phenol by OH(-), characterized by a unity-slope variation. Analyses of the direct oxidation kinetics, based on its variation with the driving force and on the determination of H/D isotope effects, ruled out a stepwise mechanism in which electron transfer is followed by the deprotonation of the initial cation radical at the benefit of a pathway in which proton and electron are transferred concertedly. Derivation of the characteristics of counterdiffusion in termolecular reactions allowed showing that the concerted process is under activation control. It is characterized by a remarkably small reorganization energy, in line with the electrochemical counterpart of the reaction, underpinning the very peculiar behavior of water as proton acceptor when it is used as the solvent.
采用激光闪光光解、氧化还原催化和停流三种实验技术，研究了在一系列电子受体提供的驱动力作用下，苯酚在纯水中氧化速率常数的变化。考虑到先前用低驱动力电子受体获得的结果，因此可以扫描超过半个电子伏特的驱动力范围。速率常数随 pH 值的变化表明，在低 pH 值下，苯酚发生直接氧化反应，速率常数不随 pH 值变化，而在 pH 值较高时，反应通过 OH-（氢氧根离子）将苯酚预先去质子化，从而发生逐步反应，其特征是斜率为 1。基于其与驱动力的变化以及 H/D 同位素效应的确定，对直接氧化动力学进行的分析排除了分步机制，在该机制中，电子转移后，初始阳离子自由基的去质子化有利于质子和电子协同转移的途径。根据分子间反应的反扩散特性，可以表明协同过程受活化控制。其特征是重组能非常小，与反应的电化学对应物一致，这解释了水作为溶剂时作为质子受体的非常特殊的行为。
