Tishchenko Oksana, Truhlar Donald G, Ceulemans Arnout, Nguyen Minh Tho
Department of Chemistry and Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455-0431, USA.
J Am Chem Soc. 2008 Jun 4;130(22):7000-10. doi: 10.1021/ja7102907. Epub 2008 May 9.
The relation between the hydrogen atom transfer (HAT) and proton-coupled electron transfer (PCET) mechanisms is discussed and is illustrated by multiconfigurational electronic structure calculations on the ArOH + R() --> ArO() + RH reactions. The key topographic features of the Born-Oppenheimer potential energy surfaces that determine the predominant reaction mechanism are the conical intersection seam of the two lowest states and reaction saddle points located on the shoulders of this seam. The saddle point corresponds to a crossing of two interacting valence bond states corresponding to the reactant and product bonding patterns, and the conical intersection corresponds to the noninteracting intersection of the same two diabatic states. The locations of mechanistically relevant conical intersection structures and relevant saddle point structures are presented for the reactions between phenol and the N- and O-centered radicals, ()NH2 and ()OOCH3. Points on the conical intersection of the ground doublet D0 and first excited doublet D1 states are found to be in close geometric and energetic proximity to the reaction saddle points. In such systems, either the HAT mechanism or both the HAT mechanism and the proton-coupled electron transfer (PCET) mechanism can take place, depending on the relative energetic accessibility of the reaction saddle points and the D0/D1 conical intersection seams. The discussion shows how the two mechanisms are related and how they blend into each other along intermediate reaction paths. The recognition that the saddle point governing the HAT mechanism is on the shoulder of the conical intersection governing the PCET mechanism is used to provide a unified view of the competition between the two mechanisms (and the blending of the two mechanisms) in terms of the prominent and connected features of the potential energy surface, namely the saddle point and the conical intersection. The character of the dual mechanism may be understood in terms of the dominant valence bond configurations of the intersecting states, which are zero-order approximations to the diabatic states.
讨论了氢原子转移（HAT）和质子耦合电子转移（PCET）机制之间的关系，并通过对ArOH + R() --> ArO() + RH反应的多组态电子结构计算进行了说明。决定主要反应机制的玻恩-奥本海默势能面的关键地形特征是两个最低态的锥形交叉缝以及位于该缝肩部的反应鞍点。鞍点对应于与反应物和产物键合模式相对应的两个相互作用的价键态的交叉，而锥形交叉对应于相同两个非绝热态的非相互作用交叉。给出了苯酚与以N和O为中心的自由基（）NH2和（）OOCH3之间反应的与机制相关的锥形交叉结构和相关鞍点结构的位置。发现基态双重态D0和第一激发双重态D1态的锥形交叉点在几何和能量上与反应鞍点非常接近。在这样的体系中，根据反应鞍点和D0/D1锥形交叉缝的相对能量可及性，要么发生HAT机制，要么同时发生HAT机制和质子耦合电子转移（PCET）机制。讨论展示了这两种机制是如何相关的，以及它们如何沿着中间反应路径相互融合。认识到控制HAT机制的鞍点位于控制PCET机制的锥形交叉的肩部，这有助于从势能面的突出且相连的特征，即鞍点和锥形交叉，对这两种机制之间的竞争（以及两种机制的融合）提供统一的观点。双重机制的特征可以根据交叉态的主导价键构型来理解，这些构型是对非绝热态的零阶近似。
