Hemelsoet Karen, Van Speybroeck Veronique, Waroquier Michel
Center for Molecular Modeling, Ghent University, Proeftuinstraat 86, 9000 Ghent, Belgium.
Chemphyschem. 2008 Nov 10;9(16):2349-58. doi: 10.1002/cphc.200800475.
The growth of polycyclic aromatic hydrocarbons (PAHs) is in many areas of combustion and pyrolysis of hydrocarbons an inconvenient side effect that warrants an extensive investigation of the underlying reaction mechanism, which is known to be a cascade of radical reactions. Herein, the focus lies on one of the key reaction classes within the coke formation process: hydrogen abstraction reactions induced by a methyl radical from methylated benzenoid species. It has been shown previously that hydrogen abstractions determine the global PAH formation rate. In particular, the influence of the polyaromatic environment on the thermodynamic and kinetic properties is the subject of a thorough exploration. Reaction enthalpies at 298 K, reaction barriers at 0 K, rate constants, and kinetic parameters (within the temperature interval 700-1100 K) are calculated by using B3LYP/6-31+G(d,p) geometries and BMK/6-311+G(3df,2p) single-point energies. This level of theory has been validated with available experimental data for the abstraction at toluene. The enhanced stability of the product benzylic radicals and its influence on the reaction enthalpies is highlighted. Corrections for tunneling effects and hindered (or free) rotations of the methyl group are taken into account. The largest spreading in thermochemical and kinetic data is observed in the series of linear acenes, and a normal reactivity-enthalpy relationship is obtained. The abstraction of a methyl hydrogen atom at one of the center rings of large methylated acenes is largely preferred. Geometrical and electronic aspects lie at the basis of this striking feature. Comparison with hydrogen abstractions leading to arylic radicals is also made.
在碳氢化合物的许多燃烧和热解领域，多环芳烃（PAHs）的生成是一个不利的副作用，这就需要对潜在的反应机理进行广泛研究，已知该反应机理是一系列自由基反应。在此，重点关注焦炭形成过程中的关键反应类别之一：甲基化苯类物种中的甲基自由基引发的氢提取反应。先前已经表明，氢提取决定了PAH的整体生成速率。特别是，多芳族环境对热力学和动力学性质的影响是深入探索的主题。利用B3LYP/6 - 31 + G(d,p)几何结构和BMK/6 - 311 + G(3df,2p)单点能量计算了298 K时的反应焓、0 K时的反应势垒、速率常数以及动力学参数（在700 - 1100 K的温度区间内）。该理论水平已通过甲苯提取的现有实验数据得到验证。突出了产物苄基自由基的增强稳定性及其对反应焓的影响。考虑了隧道效应以及甲基受阻（或自由）旋转的校正。在一系列线性并苯中观察到热化学和动力学数据的最大分散，并得到了正常的反应活性 - 焓关系。在大型甲基化并苯的中心环之一处提取甲基氢原子在很大程度上是优先的。这一显著特征的基础在于几何和电子方面。还与导致芳基自由基的氢提取进行了比较。
