Chakraborty Arindam, Zhao Yan, Lin Hai, Truhlar Donald G
Department of Chemistry and Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455-0431, USA.
J Chem Phys. 2006 Jan 28;124(4):044315. doi: 10.1063/1.2132276.
This article presents a multifaceted study of the reaction H+C(2)H(6)-->H(2)+C(2)H(5) and three of its deuterium-substituted isotopologs. First we present high-level electronic structure calculations by the W1, G3SX, MCG3-MPWB, CBS-APNO, and MC-QCISD/3 methods that lead to a best estimate of the barrier height of 11.8+/-0.5 kcal/mol. Then we obtain a specific reaction parameter for the MPW density functional in order that it reproduces the best estimate of the barrier height; this yields the MPW54 functional. The MPW54 functional, as well as the MPW60 functional that was previously parametrized for the H+CH(4) reaction, is used with canonical variational theory with small-curvature tunneling to calculate the rate constants for all four ethane reactions from 200 to 2000 K. The final MPW54 calculations are based on curvilinear-coordinate generalized-normal-mode analysis along the reaction path, and they include scaled frequencies and an anharmonic C-C bond torsion. They agree with experiment within 31% for 467-826 K except for a 38% deviation at 748 K; the results for the isotopologs are predictions since these rate constants have never been measured. The kinetic isotope effects (KIEs) are analyzed to reveal the contributions from subsets of vibrational partition functions and from tunneling, which conspire to yield a nonmonotonic temperature dependence for one of the KIEs. The stationary points and reaction-path potential of the MPW54 potential-energy surface are then used to parametrize a new kind of analytical potential-energy surface that combines a semiempirical valence bond formalism for the reactive part of the molecule with a standard molecular mechanics force field for the rest; this may be considered to be either an extension of molecular mechanics to treat a reactive potential-energy surface or a new kind of combined quantum-mechanical/molecular mechanical (QM/MM) method in which the QM part is semiempirical valence bond theory; that is, the new potential-energy surface is a combined valence bond molecular mechanics (CVBMM) surface. Rate constants calculated with the CVBMM surface agree with the MPW54 rate constants within 12% for 534-2000 K and within 23% for 200-491 K. The full CVBMM potential-energy surface is now available for use in variety of dynamics calculations, and it provides a prototype for developing CVBMM potential-energy surfaces for other reactions.
本文对反应H + C₂H₆ → H₂ + C₂H₅及其三个氘取代的同位素变体进行了多方面研究。首先，我们通过W1、G3SX、MCG3 - MPWB、CBS - APNO和MC - QCISD/3方法进行了高水平的电子结构计算，得出势垒高度的最佳估计值为11.8±0.5千卡/摩尔。然后，我们为MPW密度泛函获得了一个特定的反应参数，使其能够重现势垒高度的最佳估计值；由此得到了MPW54泛函。MPW54泛函以及先前针对H + CH₄反应进行参数化的MPW60泛函，与具有小曲率隧穿的正则变分理论一起，用于计算200至2000 K范围内所有四个乙烷反应的速率常数。最终的MPW54计算基于沿反应路径的曲线坐标广义正则模式分析，包括缩放频率和非谐C - C键扭转。在467 - 826 K范围内，其结果与实验值的偏差在31%以内，但在748 K时偏差为38%；同位素变体的结果是预测值，因为这些速率常数从未被测量过。对动力学同位素效应（KIEs）进行了分析，以揭示振动配分函数子集和隧穿的贡献，这导致其中一个KIE出现非单调的温度依赖性。然后，利用MPW54势能面的驻点和反应路径势对一种新型的分析势能面进行参数化，该势能面将分子反应部分的半经验价键形式与其余部分的标准分子力学力场相结合；这既可以被视为分子力学的扩展以处理反应势能面，也可以被视为一种新型的组合量子力学/分子力学（QM/MM）方法，其中QM部分是半经验价键理论；也就是说，新的势能面是一个组合价键分子力学（CVBMM）表面。在534 - 2000 K范围内，用CVBMM表面计算的速率常数与MPW54速率常数的偏差在12%以内，在200 - 491 K范围内偏差在23%以内。完整的CVBMM势能面现在可用于各种动力学计算，并为开发其他反应的CVBMM势能面提供了一个原型。
