Suits Arthur G
Department of Chemistry, Wayne State University, Detroit, Michigan 48202, USA.
Acc Chem Res. 2008 Jul;41(7):873-81. doi: 10.1021/ar8000734. Epub 2008 Jun 27.
The detailed description of chemical reaction rates is embodied in transition state theory (TST), now recognized as one of the great achievements of theoretical chemistry. TST employs a series of simplifying assumptions about the dynamical behavior of molecules to predict reaction rates based on a solid foundation of quantum theory and statistical mechanics. The study of unimolecular decomposition has long served as a test bed for the various assumptions of TST, foremost among which is the very notion that reactions proceed via a single well-defined transition state. Recent high-resolution ion imaging studies of formaldehyde unimolecular decomposition, in combination with quasiclassical trajectory calculations from Bowman and coworkers, have shown compelling evidence, however, for a novel pathway in unimolecular decomposition that does not proceed via the conventional transition state geometry. This "roaming" mechanism involves near dissociation to radical products followed by intramolecular abstraction to give, instead, closed shell products. This phenomenon is significant for a number of reasons: it resists easy accommodation with TST, it gives rise to a distinct, highly internally excited product state distribution, and it is likely to be a common phenomenon. These imaging studies have provided detailed insight into both the roaming dynamics and their energy-dependent branching. The dynamics are dominated by the highly exoergic long-range abstraction of H from HCO by the "roaming" hydrogen atom. The energy-dependent branching may be understood by considering the roaming behavior as being descended from the radical dissociation; that is, it grows with excess energy relative to the conventional molecular dissociation because of the larger A-factor for the radical dissociation. Recent work from several groups has identified analogous behavior in other systems. This Account explores the roaming behavior identified in imaging studies of formaldehyde and considers its implications in light of recent results for other systems.
化学反应速率的详细描述体现在过渡态理论（TST）中，该理论如今被视为理论化学的重大成就之一。TST基于量子理论和统计力学的坚实基础，采用了一系列关于分子动力学行为的简化假设来预测反应速率。单分子分解的研究长期以来一直是检验TST各种假设的试验场，其中最重要的假设是反应通过单一明确的过渡态进行。然而，最近对甲醛单分子分解的高分辨率离子成像研究，结合鲍曼及其同事的准经典轨迹计算，显示出令人信服的证据，表明单分子分解存在一条不通过传统过渡态几何结构的新途径。这种“漫游”机制涉及到接近解离为自由基产物，随后进行分子内提取，从而产生闭壳层产物。这一现象具有重要意义，原因如下：它难以与TST相契合，会产生独特的、内部高度激发的产物态分布，并且很可能是一种普遍现象。这些成像研究为漫游动力学及其能量依赖的分支提供了详细的见解。动力学主要由“漫游”氢原子从HCO中高度放热的远程提取H所主导。通过将漫游行为视为源于自由基解离，可以理解能量依赖的分支；也就是说，由于自由基解离的A因子较大，相对于传统分子解离，它会随着多余能量的增加而增加。几个研究小组最近的工作在其他系统中也发现了类似的行为。本综述探讨了在甲醛成像研究中发现的漫游行为，并根据其他系统的最新结果考虑其影响。
