Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States.
J Phys Chem B. 2012 Jan 12;116(1):571-84. doi: 10.1021/jp2091736. Epub 2011 Dec 23.
The oxidation of three phenols, which contain an intramolecular hydrogen bond to a pendent pyridine or amine group, has been shown, in a previous experimental study, to undergo concerted proton-electron transfer (CPET). In this reaction, the electron is transferred to an outer-sphere oxidant, and the proton is transferred from the oxygen to nitrogen atom. In the present study, this reaction is studied computationally using a version of Hammes-Schiffer's multistate continuum theory where CPET is formulated as a transmission frequency between neutral and cation vibrational-electronic states. The neutral and cation proton vibrational wave functions are computed from one-dimensional potential energy surfaces (PESs) for the transferring proton in a fixed heavy atom framework. The overlap integrals for these neutral/cation wave functions, considering several initial (i.e., neutral) and final (i.e., cation) vibrational states, are used to evaluate the relative rates of oxidation. The analysis is extended to heavy atom configurations with various proton donor-acceptor (i.e., O-N) distances to assess the importance of heavy atom "gating". Such changes in d(ON) dramatically affect the nature of the proton PESs and wave functions. Surprisingly, the most reactive configurations have similar donor-acceptor distances despite the large (~0.2 Å) differences in the optimized structures. These theoretical results qualitatively reproduce the experimental faster reactivity of the reaction of the pyridyl derivative 1 versus the CH(2)-pyridyl 2, but the computed factor of 5 is smaller than the experimental 10(2). The amine derivative is calculated to react similarly to 1, which does not agree with the experiments, likely due to some of the simplifying assumptions made in applying the theory. The computed kinetic isotope effects (KIEs) and their temperature dependence are in agreement with experimental results.
先前的实验研究表明，三种包含分子内氢键的酚类化合物（一个吡啶或胺基作为悬垂基团）发生协同质子-电子转移（CPET）氧化。在这个反应中，电子被转移到一个外部氧化剂上，质子从氧原子转移到氮原子。在本研究中，使用 Hammes-Schiffer 的多态连续体理论的一个版本对这个反应进行了计算研究，其中 CPET 被表述为中性和阳离子振动电子态之间的传输频率。中性和阳离子质子振动波函数是从在固定重原子框架中转移质子的一维势能表面（PES）计算得到的。考虑到几个初始（即中性）和最终（即阳离子）振动状态，这些中性/阳离子波函数的重叠积分用于评估氧化的相对速率。分析扩展到具有不同质子供体-受体（即 O-N）距离的重原子构型，以评估重原子“门控”的重要性。d（ON）的这种变化极大地影响了质子 PES 和波函数的性质。令人惊讶的是，尽管优化结构的差异很大（约 0.2 Å），但最具反应性的构型具有相似的供体-受体距离。这些理论结果定性地再现了实验中吡啶衍生物 1 比 CH(2)-吡啶 2 反应更快的反应活性，但计算出的 5 倍因子小于实验中的 10(2)。计算出的胺衍生物的反应性类似于 1，但与实验结果不一致，这可能是由于在应用理论时做了一些简化假设。计算出的动力学同位素效应（KIE）及其温度依赖性与实验结果一致。
