Hanamirian Burak, Della Libera Andrea, Pratali Maffei Luna, Cavallotti Carlo
Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, 20131Milano, Italy.
J Phys Chem A. 2023 Feb 9;127(5):1314-1328. doi: 10.1021/acs.jpca.2c08028. Epub 2023 Feb 1.
Understanding the reactivities of methylcyclopentadiene and the methylcyclopentadienyl radical is important in order to improve our comprehension of the chemical kinetics leading to the formation, decomposition, and growth of the first aromatic ring, as it has been shown that five-membered-ring species are important intermediates in the reaction kinetics of aromatic species. In this work, the rate constants of some key H-abstraction reactions from methylcyclopentadiene to produce the methylcyclopentadienyl radical and the formation of fulvene and benzene from the latter are theoretically determined. Rate constants are evaluated using the ab initio transition state theory-based master equation approach, determining structures and Hessians of all stationary points at the ωB97X-D/aug-cc-pVTZ level, energies at the CCSD(T) level extrapolated to the complete basis set limit, RRKM rate constants using conventional and variational transition state theory, and phenomenological rate constants through the solution of the one-dimensional master equation. Variational corrections are determined in both internal and Cartesian coordinates, and it is found that the choice of the coordinate system can impact the accuracy of the calculated rate constants by up to a factor of 4 for H-abstraction reactions and 2 for the unimolecular decomposition of the methylcyclopentadienyl radical. The calculated rate constants are in good agreement with the available literature data. Prompt dissociation of methylcyclopentadienyl radicals accessed following H-abstraction from methylcyclopentadiene was also investigated, and the corresponding rate constants were determined; the results show that prompt dissociation plays a key role under combustion conditions. Finally, lumping of theoretically derived rate constants to account for methylcyclopentadiene ⇄ methylcyclopentadienyl tautomerism allowed the derivation of a simplified set of rate constants suitable to be inserted into kinetic mechanisms.
了解甲基环戊二烯和甲基环戊二烯基自由基的反应活性对于提高我们对导致第一个芳香环形成、分解和生长的化学动力学的理解非常重要，因为已表明五元环物种是芳香族物种反应动力学中的重要中间体。在这项工作中，理论上确定了一些从甲基环戊二烯产生甲基环戊二烯基自由基的关键氢提取反应的速率常数，以及后者形成富烯和苯的速率常数。使用基于从头算过渡态理论的主方程方法评估速率常数，在ωB97X-D/aug-cc-pVTZ水平确定所有驻点的结构和 Hessian 矩阵，在CCSD(T)水平外推到完整基组极限的能量，使用传统和变分过渡态理论的RRKM速率常数，以及通过求解一维主方程得到的唯象速率常数。在内部坐标和笛卡尔坐标中都确定了变分校正，发现坐标系的选择对氢提取反应的计算速率常数的准确性影响高达4倍，对甲基环戊二烯基自由基的单分子分解影响高达2倍。计算得到的速率常数与现有文献数据吻合良好。还研究了从甲基环戊二烯进行氢提取后甲基环戊二烯基自由基的快速解离，并确定了相应的速率常数；结果表明，快速解离在燃烧条件下起关键作用。最后，对理论推导的速率常数进行集总以考虑甲基环戊二烯⇄甲基环戊二烯基互变异构，从而得到一组适合插入动力学机制的简化速率常数。
