Institute of Atomic and Molecular Sciences (IAMS), Academia Sinica, PO Box 23-166, Taipei 10617, Taiwan.
Department of Chemistry, Collaborative Innovation Centre of Chemistry for Energy Materials, Fudan University, Shanghai 200433, China.
Nat Chem. 2017 Dec;9(12):1175-1180. doi: 10.1038/nchem.2858. Epub 2017 Sep 11.
The transition state, which gates and modulates reactive flux, serves as the central concept in our understanding of activated reactions. The barrier height of the transition state can be estimated from the activation energy taken from thermal kinetics data or from the energetic threshold in the measured excitation function (the dependence of reaction cross-sections on initial collision energies). However, another critical and equally important property, the angle-dependent barrier to reaction, has not yet been amenable to experimental determination until now. Here, using the benchmark reaction of Cl + CHD(v = 1) as an example, we show how to map this anisotropic property of the transition state as a function of collision energy from the preferred reactant bond alignment of the backward-scattered products-the imprints of small impact-parameter collisions. The deduced bend potential at the transition state agrees with ab initio calculations. We expect that the method should be applicable to many other direct reactions with a collinear barrier.
过渡态是激活反应理解的核心概念，它起到了门控和调节反应通量的作用。过渡态的势垒高度可以从热动力学数据中的活化能或测量激发函数中的能量阈值（反应截面随初始碰撞能的变化）中估算得到。然而，另一个关键且同样重要的性质，即反应的角度相关势垒，直到现在还无法通过实验来确定。在这里，我们以 Cl + CHD(v = 1)的基准反应为例，展示了如何通过后向散射产物中优先反应物键的取向——小碰撞参数碰撞的印记，来映射过渡态的各向异性性质作为碰撞能的函数。在过渡态处推断出的弯曲势与从头算计算结果一致。我们预计该方法应该适用于许多具有共线势垒的其他直接反应。
