Yang Xueming, Zhang Dong H
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute ofChemical Physics, Chinese Academy of Sciences, Dalian, China.
Acc Chem Res. 2008 Aug;41(8):981-9. doi: 10.1021/ar700258g.
[Reaction: see text]. The concept of transition state has played a crucial role in the field of chemical kinetics and reaction dynamics. Resonances in the transition state region are important in many chemical reactions at reaction energies near the thresholds. Detecting and characterizing isolated reaction resonances, however, have been a major challenge in both experiment and theory. In this Account, we review the most recent developments in the study of reaction resonances in the benchmark F + H 2 --> HF + H reaction. Crossed molecular beam scattering experiments on the F + H 2 reaction have been carried out recently using the high-resolution, highly sensitive H-atom Rydberg tagging technique with HF rovibrational states almost fully resolved. Pronounced forward scattering for the HF (nu' = 2) product has been observed at the collision energy of 0.52 kcal/mol in the F + H 2 (j = 0) reaction. Quantum dynamical calculations based on two new potential energy surfaces, the Xu-Xie-Zhang (XXZ) surface and the Fu-Xu-Zhang (FXZ) surface, show that the observed forward scattering of HF (nu' = 2) in the F + H 2 reaction is caused by two Feshbach resonances (the ground resonance and first excited resonance). More interestingly, the pronounced forward scattering of HF (nu' = 2) at 0.52 kcal/mol is enhanced considerably by the constructive interference between the two resonances. In order to probe the resonance potential more accurately, the isotope substituted F + HD --> HF + D reaction has been studied using the D-atom Rydberg tagging technique. A remarkable and fast changing dynamical picture has been mapped out in the collision energy range of 0.3-1.2 kcal/mol for this reaction. Quantum dynamical calculations based on the XXZ surface suggest that the ground resonance on this potential is too high in comparison with the experimental results of the F + HD reaction. However, quantum scattering calculations on the FXZ surface can reproduce nearly quantitatively the resonance picture of the F + HD reaction observed in the experiment. It is clear that the dynamics of the F + HD reaction below the threshold was dominated by the ground resonance state. Furthermore, the forward scattering HF (nu' = 3) channel from the F + H 2 ( j = 0) reaction was investigated and was attributed mainly to a slow-down mechanism over the centrifugal exit barrier, with small contributions from a shape resonance mechanism in a narrow collision energy range. A striking effect of the reagent rotational excitation on resonance was also observed in F + H 2 ( j = 1), in comparison with F + H 2 ( j = 0). From these concerted experimental and theoretical studies, a clear physical picture of the reaction resonances in this benchmark reaction has emerged, providing a textbook example of dynamical resonances in elementary chemical reactions.
[反应：见正文]。过渡态的概念在化学动力学和反应动力学领域发挥了关键作用。在接近阈值的反应能量下，过渡态区域的共振在许多化学反应中都很重要。然而，检测和表征孤立的反应共振在实验和理论上都是一项重大挑战。在本综述中，我们回顾了基准反应F + H₂→HF + H中反应共振研究的最新进展。最近，利用高分辨率、高灵敏度的氢原子里德堡标记技术，对F + H₂反应进行了交叉分子束散射实验，几乎完全分辨出了HF的振转态。在F + H₂（j = 0）反应中，在0.52 kcal/mol的碰撞能量下，观察到HF（ν' = 2）产物有明显的前向散射。基于两个新的势能面，即徐-谢-张（XXZ）面和傅-徐-张（FXZ）面的量子动力学计算表明，在F + H₂反应中观察到的HF（ν' = 2）的前向散射是由两个费什巴赫共振（基态共振和第一激发共振）引起的。更有趣的是，在0.52 kcal/mol时，HF（ν' = 2）明显的前向散射因两个共振之间的相长干涉而大大增强。为了更准确地探测共振势，使用氘原子里德堡标记技术研究了同位素取代反应F + HD→HF + D。在该反应0.3 - 1.2 kcal/mol的碰撞能量范围内，描绘出了一幅显著且快速变化的动力学图景。基于XXZ面的量子动力学计算表明，与F + HD反应的实验结果相比，该势能面上的基态共振过高。然而，基于FXZ面的量子散射计算几乎可以定量地重现实验中观察到的F + HD反应的共振图景。很明显，F + HD反应低于阈值时的动力学主要由基态共振态主导。此外，对F + H₂（j = 0）反应的前向散射HF（ν' = 3）通道进行了研究，其主要归因于离心出口势垒上的减速机制，在狭窄的碰撞能量范围内，形状共振机制的贡献较小。与F + H₂（j = 0）相比，在F + H₂（j = 1）中还观察到了反应物转动激发对共振的显著影响。通过这些协同的实验和理论研究，在这个基准反应中反应共振的清晰物理图景已经浮现，为基本化学反应中的动力学共振提供了一个典型例子。
