Cao Jianwei, Zhang Zhijun, Zhang Chunfang, Liu Kun, Wang Manhui, Bian Wensheng
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
Proc Natl Acad Sci U S A. 2009 Aug 11;106(32):13180-5. doi: 10.1073/pnas.0903934106. Epub 2009 Jul 29.
This work elucidates new atomic-level mechanisms that may be common in a range of chemical reactions, and our findings are important for the understanding of the nature of polyatomic abstraction and exchange reactions. A global 12-dimensional ab initio potential energy surface (PES), which describes both H+SiH(4) abstraction and exchange reactions is constructed, based on the modified Shepard interpolation method and UCCSD(T)/cc-pVQZ energy calculations at 4,015 geometries. This PES has a classical barrier height of 5.35 kcal/mol for abstraction (our best estimate is 5.35 +/- 0.15 kcal/mol from extensive ab initio calculations), and an exothermicity of -13.12 kcal/mol, in excellent agreement with experiment. Quasiclassical trajectory calculations on this new PES reveal interesting features of detailed dynamical quantities and underlying new mechanisms. Our calculated product angular distributions for exchange are in the forward hemisphere with a tail sideways, and are attributed to the combination of three mechanisms: inversion, torsion-tilt, and side-inversion. With increase of collision energy our calculated angular distributions for abstraction first peak at backward scattering and then shift toward smaller scattering angles, which is explained by a competition between rebound and stripping mechanisms; here stripping is seen at much lower energies, but is conceptually similar to what was observed in the reaction of H+CD(4) by Zare and coworkers [Camden JP, et al. (2005) J Am Chem Soc 127:11898-11899]. Each of these atomic-level mechanisms is confirmed by direct examination of trajectories, and two of them (torsion-tilt and side-inversion) are proposed and designated in this work.
这项工作阐明了一系列化学反应中可能常见的新的原子水平机制，我们的发现对于理解多原子抽象和交换反应的本质很重要。基于改进的谢泼德插值方法以及在4015个几何构型下的UCCSD(T)/cc-pVQZ能量计算，构建了一个描述H+SiH₄抽象和交换反应的全局12维从头算势能面（PES）。该PES对于抽象反应的经典势垒高度为5.35千卡/摩尔（我们通过广泛的从头算计算得出的最佳估计值为5.35±0.15千卡/摩尔），放热为-13.12千卡/摩尔，与实验结果非常吻合。在这个新的PES上进行的准经典轨迹计算揭示了详细动力学量的有趣特征和潜在的新机制。我们计算得到的交换反应产物角分布在前半球且有一个侧向的尾部，这归因于三种机制的组合：反转、扭转倾斜和侧向反转。随着碰撞能量的增加，我们计算得到的抽象反应角分布首先在向后散射处达到峰值，然后向较小的散射角移动，这可以通过反弹和剥离机制之间的竞争来解释；这里剥离在低得多的能量下就可以看到，但在概念上与扎雷及其同事在H+CD₄反应中观察到的情况相似[卡姆登JP等人（2005年）《美国化学会志》127:11898 - 11899]。这些原子水平机制中的每一种都通过对轨迹的直接检查得到了证实，其中两种（扭转倾斜和侧向反转）是在这项工作中提出并命名的。
