Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, California 90089-1453, USA.
J Phys Chem A. 2012 Jan 12;116(1):319-32. doi: 10.1021/jp209360u. Epub 2011 Dec 27.
The role of quantum tunneling in hydrogen shift in linear heptyl radicals is explored using multidimensional, small-curvature tunneling method for the transmission coefficients and a potential energy surface computed at the CBS-QB3 level of theory. Several one-dimensional approximations (Wigner, Skodje and Truhlar, and Eckart methods) were compared to the multidimensional results. The Eckart method was found to be sufficiently accurate in comparison to the small-curvature tunneling results for a wide range of temperature, but this agreement is in fact fortuitous and caused by error cancellations. High-pressure limit rate constants were calculated using the transition state theory with treatment of hindered rotations and Eckart transmission coefficients for all hydrogen-transfer isomerizations in n-pentyl to n-octyl radicals. Rate constants are found in good agreement with experimental kinetic data available for n-pentyl and n-hexyl radicals. In the case of n-heptyl and n-octyl, our calculated rates agree well with limited experimentally derived data. Several conclusions made in the experimental studies of Tsang et al. (Tsang, W.; McGivern, W. S.; Manion, J. A. Proc. Combust. Inst. 2009, 32, 131-138) are confirmed theoretically: older low-temperature experimental data, characterized by small pre-exponential factors and activation energies, can be reconciled with high-temperature data by taking into account tunneling; at low temperatures, transmission coefficients are substantially larger for H-atom transfers through a five-membered ring transition state than those with six-membered rings; channels with transition ring structures involving greater than 8 atoms can be neglected because of entropic effects that inhibit such transitions. The set of computational kinetic rates were used to derive a general rate rule that explicitly accounts for tunneling. The rate rule is shown to reproduce closely the theoretical rate constants.
使用多维小曲率隧穿方法计算传输系数和 CBS-QB3 理论计算的势能面，研究了量子隧穿在直链庚基自由基中氢迁移的作用。将几种一维近似（Wigner、Skodje 和 Truhlar 以及 Eckart 方法）与多维结果进行了比较。发现 Eckart 方法与小曲率隧穿结果相比，在较宽的温度范围内足够准确，但这种一致性实际上是偶然的，是由误差抵消造成的。对于 n-戊基到 n-辛基自由基的所有氢转移异构化反应，使用过渡态理论并考虑受阻旋转和 Eckart 传输系数计算了高压极限速率常数。对于 n-戊基和 n-己基自由基，发现速率常数与现有的实验动力学数据非常吻合。对于 n-庚基和 n-辛基，我们的计算速率与有限的实验推导数据非常吻合。Tsang 等人在实验研究中得出的几个结论得到了理论上的证实：较旧的低温实验数据，其特征是较小的指前因子和活化能，可以通过考虑隧穿来与高温数据相协调；在低温下，对于通过五元环过渡态的 H 原子转移，传输系数比六元环的传输系数大得多；由于熵效应抑制了这种跃迁，因此可以忽略涉及大于 8 个原子的过渡环结构的通道。一组计算的动力学速率被用来推导出一个明确考虑隧穿的通用速率规则。该速率规则被证明可以很好地再现理论速率常数。
