Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States.
J Phys Chem A. 2011 Dec 22;115(50):14370-81. doi: 10.1021/jp208347j. Epub 2011 Nov 28.
We present a statistical theory for the effect of roaming pathways on product branching fractions in both unimolecular and bimolecular reactions. The analysis employs a separation into three distinct steps: (i) the formation of weakly interacting fragments in the long-range/van der Waals region of the potential via either partial decomposition (for unimolecular reactants) or partial association (for bimolecular reactants), (ii) the roaming step, which involves the reorientation of the fragments from one region of the long-range potential to another, and (iii) the abstraction, addition, and/or decomposition from the long-range region to yield final products. The branching between the roaming induced channel(s) and other channels is obtained from a steady-state kinetic analysis for the two (or more) intermediates in the long-range region of the potential. This statistical theory for the roaming-induced product branching is illustrated through explicit comparisons with reduced dimension trajectory simulations for the decompositions of H(2)CO, CH(3)CHO, CH(3)OOH, and CH(3)CCH. These calculations employ high-accuracy analytic potentials obtained from fits to wide-ranging CASPT2 ab initio electronic structure calculations. The transition-state fluxes for the statistical theory calculations are obtained from generalizations of the variable reaction coordinate transition state theory approach. In each instance, at low energy the statistical analysis accurately reproduces the branching obtained from the trajectory simulations. At higher energies, e.g., above 1 kcal/mol, increasingly large discrepancies arise, apparently due to a dynamical biasing toward continued decomposition of the incipient molecular fragments (for unimolecular reactions). Overall, the statistical theory based kinetic analysis is found to provide a useful framework for interpreting the factors that determine the significance of roaming pathways in varying chemical environments.
我们提出了一种统计理论，用于研究 roaming 途径对单分子和双分子反应中产物分支比的影响。该分析采用了三个不同步骤的分离：（i）通过部分分解（对于单分子反应物）或部分缔合（对于双分子反应物），在势能的长程/范德华区域中形成弱相互作用的碎片，（ii）roaming 步骤，涉及碎片从长程势能的一个区域到另一个区域的重新取向，（iii）从长程区域中提取、添加和/或分解以生成最终产物。 roaming 诱导通道与其他通道之间的分支比是通过对势能长程区域中的两个（或更多）中间体进行稳态动力学分析获得的。通过与 H(2)CO、CH(3)CHO、CH(3)OOH 和 CH(3)CCH 的分解的降维轨迹模拟的明确比较，说明了 roaming 诱导产物分支的这种统计理论。这些计算采用了从广泛的 CASPT2 从头算电子结构计算拟合得到的高精度分析势能。统计理论计算的过渡态通量是通过变量反应坐标过渡态理论方法的推广获得的。在每种情况下，在低能下，统计分析准确地再现了轨迹模拟得到的分支比。在更高的能量下，例如，高于 1 kcal/mol，会出现越来越大的差异，这显然是由于初始分子碎片继续分解的动力学偏向（对于单分子反应）。总体而言，基于统计理论的动力学分析被发现为解释在不同化学环境中 roaming 途径的重要性的因素提供了有用的框架。
