Beste Ariana, Buchanan A C, Britt Phillip F, Hathorn Bryan C, Harrison Robert J
Computer Science and Mathematics Division, Oak Ridge National Laboratory, Bethel Valley Road, Oak Ridge, Tennessee 37831-6367, USA.
J Phys Chem A. 2007 Dec 6;111(48):12118-26. doi: 10.1021/jp075861+. Epub 2007 Nov 9.
We calculated an overall alpha/beta-selectivity for the pyrolysis of phenethyl phenyl ether as a composite of the alpha/beta-selectivities in the hydrogen abstraction reactions by the phenoxyl and by the benzyl radical that is in excellent agreement with experiment. The difference between the individual selectivities for these radicals is explained by analyzing the electronic structure of the transition states. Spin delocalization of the single electron favors the alpha-pathways. An opposing effect occurs for polarized transition states, such as the transition states for the hydrogen abstraction by the electrophilic phenoxyl radical, where the adjacent ether oxygen in phenethyl phenyl ether stabilizes the beta-transition states. These results indicate that theory will be able to provide excellent predictions of alpha/beta-product selectivities for more complicated lignin model compounds bearing multiple substituents. We have developed a scheme to predict alpha/beta-product selectivities in the pyrolysis of model compounds for the beta-ether linkage in lignin. The approach is based on computation of the relative rate constant, which profits from error cancellation in the individual rate constants. The Arrhenius prefactors depend strongly on the description of the low-frequency modes for which anharmonic contributions are important. We use density functional theory in combination with transition-state theory in this analysis. Diagonal anharmonic effects for individual low-frequency modes are included by employing a second-order Wigner-Kirkwood expansion in a semiclassical expression for the vibrational partition function. The composite alpha/beta-product selectivity is obtained by applying quasi-steady-state kinetic analysis for the intermediate radicals.
我们计算了苯乙基苯基醚热解的总体α/β选择性，它是苯氧基和苄基自由基氢提取反应中α/β选择性的综合结果，与实验结果高度吻合。通过分析过渡态的电子结构来解释这些自由基各自选择性之间的差异。单电子的自旋离域有利于α途径。对于极化的过渡态，如亲电苯氧基自由基氢提取的过渡态，会出现相反的效应，其中苯乙基苯基醚中相邻的醚氧稳定了β过渡态。这些结果表明，理论将能够为更复杂的带有多个取代基的木质素模型化合物的α/β产物选择性提供出色的预测。我们已经开发了一种方案来预测木质素中β-醚键模型化合物热解中的α/β产物选择性。该方法基于相对速率常数的计算，这得益于各个速率常数中的误差抵消。阿仑尼乌斯前因子强烈依赖于低频模式的描述，对于这些模式，非谐贡献很重要。在该分析中，我们使用密度泛函理论结合过渡态理论。通过在振动配分函数的半经典表达式中采用二阶维格纳 - 柯克伍德展开来包含各个低频模式的对角非谐效应。通过对中间自由基应用准稳态动力学分析获得复合α/β产物选择性。
