Karlsruhe Institute of Technology, Engler-Bunte-Institut, Karlsruhe, Germany.
J Phys Chem A. 2011 Oct 27;115(42):11897-914. doi: 10.1021/jp2078067. Epub 2011 Sep 23.
Alkyl substituted aromatics are present in fuels and in the environment because they are major intermediates in the oxidation or combustion of gasoline, jet, and other engine fuels. The major reaction pathways for oxidation of this class of molecules is through loss of a benzyl hydrogen atom on the alkyl group via abstraction reactions. One of the major intermediates in the combustion and atmospheric oxidation of the benzyl radicals is benzaldehyde, which rapidly loses the weakly bound aldehydic hydrogen to form a resonance stabilized benzoyl radical (C6H5C(•)═O). A detailed study of the thermochemistry of intermediates and the oxidation reaction paths of the benzoyl radical with dioxygen is presented in this study. Structures and enthalpies of formation for important stable species, intermediate radicals, and transition state structures resulting from the benzoyl radical +O2 association reaction are reported along with reaction paths and barriers. Enthalpies, ΔfH298(0), are calculated using ab initio (G3MP2B3) and density functional (DFT at B3LYP/6-311G(d,p)) calculations, group additivity (GA), and literature data. Bond energies on the benzoyl and benzoyl-peroxy systems are also reported and compared to hydrocarbon systems. The reaction of benzoyl with O2 has a number of low energy reaction channels that are not currently considered in either atmospheric chemistry or combustion models. The reaction paths include exothermic, chain branching reactions to a number of unsaturated oxygenated hydrocarbon intermediates along with formation of CO2. The initial reaction of the C6H5C(•)═O radical with O2 forms a chemically activated benzoyl peroxy radical with 37 kcal mol(-1) internal energy; this is significantly more energy than the 21 kcal mol(-1) involved in the benzyl or allyl + O2 systems. This deeper well results in a number of chemical activation reaction paths, leading to highly exothermic reactions to phenoxy radical + CO2 products.
取代芳烃存在于燃料和环境中，因为它们是汽油、喷气燃料和其他发动机燃料氧化或燃烧的主要中间体。该类分子氧化的主要反应途径是通过烷基上的苄基氢原子通过提取反应失去。在苄基自由基的燃烧和大气氧化过程中，主要中间体之一是苯甲醛，它迅速失去弱结合的醛氢，形成共振稳定的苯甲酰自由基（C6H5C(•)=O）。本研究详细研究了中间体的热化学和苯甲酰自由基与氧气的氧化反应途径。报告了重要稳定物种、中间自由基以及苯甲酰自由基+O2缔合反应产生的过渡态结构的结构和生成焓，以及反应路径和势垒。使用从头算（G3MP2B3）和密度泛函（B3LYP/6-311G(d,p)）计算、基团加性（GA）和文献数据计算焓，ΔfH298(0)。还报告了苯甲酰和苯甲酰过氧体系的键能，并与碳氢化合物体系进行了比较。苯甲酰与 O2 的反应有许多低能反应通道，目前在大气化学或燃烧模型中都没有考虑到。反应途径包括放热、支链反应，生成许多不饱和含氧碳氢化合物中间体，同时形成 CO2。C6H5C(•)=O 自由基与 O2 的初始反应形成具有 37 kcal mol(-1)内部能的化学活化苯甲酰过氧自由基；这比苄基或烯丙基+O2 体系中涉及的 21 kcal mol(-1)要多得多。这种更深的势阱导致了许多化学活化反应途径，导致高度放热的反应生成苯氧基自由基+CO2 产物。
