Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria, Australia.
Phys Chem Chem Phys. 2012 Dec 14;14(46):16143-54. doi: 10.1039/c2cp42635h.
The resonance stabilized benzyl radical is an important intermediate in the combustion of aromatic hydrocarbons and in polycyclic aromatic hydrocarbon (PAH) formation in flames. Despite being a free radical, benzyl is relatively stable in thermal, oxidizing environments, and is predominantly removed through bimolecular reactions with open-shell species other than O(2). In this study the reaction of benzyl with ground-state atomic oxygen, O((3)P), is examined using quantum chemistry and statistical reaction rate theory. C(7)H(7)O energy surfaces are generated at the G3SX level, and include several novel pathways. Transition state theory is used to describe elementary reaction kinetics, with canonical variational transition state theory applied for barrierless O atom association with benzyl. Apparent rate constants and branching ratios to different product sets are obtained as a function of temperature and pressure from solving the time-dependent master equation, with RRKM theory for microcanonical k(E). These simulations indicate that the benzyl + O reaction predominantly forms the phenyl radical (C(6)H(5)) plus formaldehyde (HCHO), with lesser quantities of the C(7)H(6)O products benzaldehyde, ortho-quinone methide, and para-quinone methide (+H), along with minor amounts of the formyl radical (HCO) + benzene. Addition of O((3)P) to the methylene site in benzyl produces a highly vibrationally excited C(7)H(7)O* adduct, the benzoxyl radical, which can β-scission to benzaldehyde + H and phenyl + HCHO. In order to account for the experimental observation of benzene as the major reaction product, a roaming radical mechanism is proposed that converts the nascent products phenyl and HCHO to benzene + HCO. Oxygen atom addition at the ortho and para ring sites in benzyl, which has not been previously considered, is shown to lead to the quinone methides + H; these species are less-stable isomers of benzaldehyde that are proposed as important combustion intermediates, but are yet to be identified experimentally. Franck-Condon simulations of the benzaldehyde, o-quinone methide, and p-quinone methide photoelectron spectra suggest that these C(7)H(6)O isomers could be distinguished using tunable VUV photoionization mass spectrometry.
共振稳定的苄基自由基是芳香烃燃烧和火焰中环多芳烃（PAH）形成中的重要中间体。尽管是自由基，但苄基在热、氧化环境中相对稳定，主要通过与除 O(2)以外的开壳物种的双分子反应去除。在这项研究中，使用量子化学和统计反应速率理论研究了苄基与基态氧原子 O((3)P)的反应。在 G3SX 水平上生成了 C(7)H(7)O 能面，其中包括几个新途径。过渡态理论用于描述基本反应动力学，对于无势垒 O 原子与苄基的缔合，应用正则变分过渡态理论。通过求解时变主方程，以温度和压力为函数，从获得不同产物集的表观速率常数和分支比，微正则 k(E)的 RRKM 理论用于计算。这些模拟表明，苄基+O 反应主要形成苯基自由基（C(6)H(5)）加甲醛（HCHO），其次是 C(7)H(6)O 产物苯甲醛、邻醌甲叉、对醌甲叉（+H），以及少量的甲酰自由基（HCO）+苯。O((3)P)加成到苄基的亚甲基位点生成高度振动激发的 C(7)H(7)O*加合物，苯氧自由基，其可以β-断裂生成苯甲醛+H 和苯基+HCHO。为了解释实验观察到的苯是主要反应产物，提出了一个漫游自由基机制，将新生成的产物苯基和 HCHO 转化为苯+HCO。以前没有考虑过的苄基邻位和对位环位点的氧原子加成导致了醌甲叉+H；这些物质是苯甲醛的不稳定异构体，被提议作为重要的燃烧中间体，但尚未在实验中得到证实。苯甲醛、邻醌甲叉和对醌甲叉光电子能谱的 Franck-Condon 模拟表明，这些 C(7)H(6)O 异构体可以使用可调谐 VUV 光致电离质谱来区分。
