Direct Kinetics and Product Measurement of Phenyl Radical + Ethylene

The phenyl + ethylene (C6H5 + C2H4) reaction network was explored experimentally and theoretically to understand the temperature dependence of the reaction kinetics and product distribution under various temperature and pressure conditions. The flash photolysis apparatus combining laser absorbance spectroscopy (LAS) and time-resolved molecular beam mass spectrometry (MBMS) was used to study reactions on the C8H9 potential energy surface (PES). In LAS experiments, 505.3 nm laser light selectively probed C6H5 decay, and we measured the total C6H5 consumption rate coefficients in the intermediate temperature region (400–800 K), which connects previous experiments performed in high-temperature (pyrolysis) and low-temperature (cavity-ring-down methods) regions. From the quantum chemistry calculations by Tokmakov and Lin using the G2M­(RCC5)//B3LYP method, we constructed a kinetic model and estimated phenomenological pressure-dependent rate coefficients, k(T, P), with the Arkane package in the reaction mechanism generator. The MBMS experiments, performed at 600–800 K and 10–50 Torr, revealed three major product peaks: m/z = 105 (adducts, mostly 2-phenylethyl radical, but also 1-phenylethyl radical, ortho-ethyl phenyl radical, and a spiro-fused ring radical), 104 (styrene, co-product with a H atom), and 78 (benzene, co-product with C2H3 radical). Product branching ratios were predicted by the model and validated by experiments for the first time. At 600 K and 10 Torr, the yield ratio of the H-abstraction reaction (forming benzene + C2H3) is measured to be 1.1% and the H-loss channel (styrene + H) has a 2.5% yield ratio. The model predicts 1.0% for H-abstraction and 2.3% for H-loss, which is within the experimental error bars. The branching ratio and formation of styrene increase at high temperature due to the favored formally direct channel (1.0% at 600 K and 10 Torr, 5.8% at 800 K and 10 Torr in the model prediction) and the faster β-scission reactions of C8H9 isomers. The importance of pressure dependence in kinetics is verified by the increase in the yield of the stabilized adduct from radical addition from 80.2% (800 K, 10 Torr) to 88.9% (800 K, 50 Torr), at the expense of styrene + H. The pressure-dependent model developed in this work is well validated by the LAS and MBMS measurements and gives a complete picture of the C6H5 + C2H4 reaction.本研究通过实验与理论结合的手段，对苯基+乙烯（C₆H₅ + C₂H₄）反应网络开展探索，以明晰不同温度与压力条件下，反应动力学及产物分布的温度依赖性。本研究采用结合激光吸收光谱（Laser Absorbance Spectroscopy, LAS）与时间分辨分子束质谱（Time-resolved Molecular Beam Mass Spectrometry, MBMS）的闪速光解装置，针对C₈H₉势能面（Potential Energy Surface, PES）上的反应开展研究。在激光吸收光谱实验中，研究人员使用505.3 nm激光选择性探测苯基（C₆H₅）的衰变过程，并测定了中温区间（400–800 K）内苯基的总消耗速率系数，该结果衔接了此前分别在高温（热解实验）与低温（腔衰荡光谱法）区间开展的相关研究。基于Tokmakov与Lin采用G2M(RCC5)//B3LYP方法完成的量子化学计算，本研究构建了动力学模型，并借助反应机理生成器中的Arkane软件包，估算得到现象学压力依赖型速率系数k(T, P)。在600–800 K、10–50 Torr条件下开展的分子束质谱实验，共观测到三类主要产物峰：质荷比（mass-to-charge ratio, m/z）为105的加合物（主要成分为2-苯乙基自由基，同时还包含1-苯乙基自由基、邻乙基苯基自由基与稠合螺环自由基）、m/z=104的苯乙烯（与氢原子为共同产物），以及m/z=78的苯（与C₂H₃自由基为共同产物）。本研究首次通过动力学模型预测了产物分支比，并通过实验完成了验证。在600 K、10 Torr条件下，经实验测得夺氢反应（生成苯+C₂H₃）的产率为1.1%，氢损失通道（生成苯乙烯+H）的产率为2.5%；模型预测的夺氢反应产率为1.0%，氢损失通道产率为2.3%，二者均处于实验误差范围内。由于形式上的直接反应通道（模型预测：600 K、10 Torr下占比1.0%，800 K、10 Torr下占比5.8%）更易发生，且C₈H₉异构体的β-断裂反应速率更快，苯乙烯的产物分支比与生成量随温度升高而显著提升。压力对反应动力学的影响得到了验证：自由基加成生成的稳定加合物产率从800 K、10 Torr下的80.2%提升至800 K、50 Torr下的88.9%，而苯乙烯+H通道的产率则相应降低。本研究构建的压力依赖型动力学模型，经激光吸收光谱与分子束质谱实验数据充分验证，完整描绘了C₆H₅ + C₂H₄反应的全貌。