Silva Gabriel da, Bozzelli Joseph W
Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, USA.
J Phys Chem A. 2007 Aug 16;111(32):7987-94. doi: 10.1021/jp073335c. Epub 2007 Jul 24.
o-Quinone methide (o-QM), or 6-methylene-2,4-cyclohexadiene-1-one, has been identified as an important intermediate in lignin and alkyl benzene combustion, and the thermal decomposition of o-QM is therefore relevant to the combustion of transportation fuels (which contain toluene) and of biomass and low-rank coals (which contain lignin). We present a comprehensive reaction mechanism for the unimolecular conversion of o-QM to the reaction intermediates tropone and fulvene, calculated using theoretical quantum chemical techniques. Enthalpies of formation for all reactants, products, and intermediates are calculated using the CBS-QB3 theoretical method. Transition states are determined with the CBS-QB3 method, which we use to obtain rate constants as a function of temperature from transition-state theory, with Wigner tunneling corrections applied to hydrogen-shift reactions. Barrier heights are also calculated with the BB1K density functional theory (DFT) method for thermochemical kinetics. Reaction pathways are identified leading to tropone (which rapidly decomposes to benzene + CO) and to fulvene + CO, via initial hydrogen transfer to 2-hydroxyphenylcarbene and via ring opening to 1,3,5,6-heptatetraen-1-one, respectively. Quantum Rice-Ramsperger-Kassel (QRRK) theory analysis of the reaction kinetics indicates that the dominant reaction pathway is formation of tropone via 2-hydroxyphenylcarbene; the formation of fulvene + CO via initial ring opening constitutes a secondary pathway, which becomes more important with increasing temperature. Our calculations, using BB1K barrier heights, yield the rate equation k(T) [s(-1)] = 2.64 x 10(14) exp(-35.9/T [K]) for o-QM decomposition, which is in relatively good agreement with the experimental rate equation. Calculations provide an apparent activation energy of 71.3 kcal mol(-1), versus 67.2 kcal mol(-1) from experiment.
邻醌甲基化物（o-QM），即6-亚甲基-2,4-环己二烯-1-酮，已被确定为木质素和烷基苯燃烧过程中的一种重要中间体，因此o-QM的热分解与运输燃料（含有甲苯）以及生物质和低阶煤（含有木质素）的燃烧相关。我们采用理论量子化学技术，给出了o-QM单分子转化为反应中间体环庚三烯酮和富烯的综合反应机理。使用CBS-QB3理论方法计算了所有反应物、产物和中间体的生成焓。用CBS-QB3方法确定过渡态，我们用该方法根据过渡态理论获得速率常数随温度的变化关系，并对氢转移反应应用维格纳隧穿校正。还用BB1K密度泛函理论（DFT）方法计算了热化学动力学的势垒高度。确定了分别通过最初氢转移至2-羟基苯卡宾以及开环至1,3,5,6-庚四烯-1-酮生成环庚三烯酮（其迅速分解为苯 + CO）和富烯 + CO的反应途径。对反应动力学的量子 Rice-Ramsperger-Kassel（QRRK）理论分析表明，主要反应途径是通过2-羟基苯卡宾生成环庚三烯酮；最初开环生成富烯 + CO构成次要途径，该途径随温度升高变得更为重要。我们使用BB1K势垒高度进行的计算得出o-QM分解的速率方程k(T) [s⁻¹] = 2.64×10¹⁴ exp(-35.9/T [K])，这与实验速率方程相对吻合。计算得出的表观活化能为71.3 kcal mol⁻¹，而实验值为67.2 kcal mol⁻¹。
