Schleier D, Constantinidis P, Faßheber N, Fischer I, Friedrichs G, Hemberger P, Reusch E, Sztáray B, Voronova K
Institute of Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany.
Phys Chem Chem Phys. 2018 Apr 25;20(16):10721-10731. doi: 10.1039/c7cp07893e.
The kinetics of the combustion-relevant reaction of the allyl radical, a-C3H5, with molecular oxygen has been studied in a flow tube reactor at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source storage ring, using the CRF-PEPICO (Combustion Reactions Followed by Photoelectron Photoion Coincidence Spectroscopy) setup. The ability to measure threshold photoelectron spectra enables a background-free detection of reactive species as well as an isomer-specific analysis of reaction products. Allyl was generated by direct photodissociation of allyl iodide at 266 nm and 213 nm and indirectly by the reaction of propene with Cl atoms, which were generated by photolysis from oxalyl chloride at 266 nm. Experiments were conducted at room temperature at low pressures between 0.8 and 3 mbar using Ar as the buffer gas and with excess O2 to maintain nearly pseudo-first-order reaction conditions. Whereas allyl was detected by photoionisation using synchrotron radiation, the main reaction product allyl peroxy was not observed due to dissociative ionisation of this weakly bound species. From the concentration-time profiles of the allyl signal, second-order rate constants between 1.35 × 1011 cm3 mol-1 s-1 at 0.8 mbar and 1.75 × 1011 cm3 mol-1 s-1 at 3 mbar were determined. The rates obtained for the different allyl radical generation schemes agree well with each other, but are about a factor of 2 higher than the ones reported previously using He as a buffer gas. The discrepancy is partly attributed to the higher collision efficiency of Ar causing a varying fall-off behavior. When allyl is produced by the reaction of propene with Cl atom, an unexpected product is observed at m/z = 68, which was identified as 1,3-butadienal in the threshold photoelectron spectrum. It is formed in a secondary reaction of allyl with the OCCl radical, which is generated in the 266 nm photolysis of oxalyl chloride.
在瑞士光源储存环的真空紫外（VUV）光束线的流动管反应器中，使用CRF - PEPICO（光电子光离子符合光谱法跟踪燃烧反应）装置研究了烯丙基自由基（α-C₃H₅）与分子氧的燃烧相关反应动力学。测量阈值光电子能谱的能力使得能够无背景地检测反应物种，并对反应产物进行异构体特异性分析。烯丙基通过在266 nm和213 nm处碘化烯丙基的直接光解离以及间接通过丙烯与氯原子的反应产生，氯原子由266 nm处草酰氯的光解产生。实验在室温下于0.8至3毫巴的低压下进行，使用氩气作为缓冲气体并加入过量的氧气以维持近伪一级反应条件。虽然使用同步辐射通过光电离检测到了烯丙基，但由于这种弱结合物种的解离电离，未观察到主要反应产物烯丙基过氧自由基。根据烯丙基信号的浓度-时间曲线，确定了在0.8毫巴下二级速率常数为1.35×10¹¹ cm³ mol⁻¹ s⁻¹，在3毫巴下为1.75×10¹¹ cm³ mol⁻¹ s⁻¹。通过不同烯丙基自由基生成方案获得的速率彼此吻合良好，但比先前使用氦气作为缓冲气体报道的速率约高2倍。这种差异部分归因于氩气较高的碰撞效率导致不同的衰减行为。当通过丙烯与氯原子的反应产生烯丙基时，在m/z = 68处观察到一种意外产物，在阈值光电子能谱中被鉴定为1,3 - 丁二烯醛。它是在烯丙基与OCCl自由基的二级反应中形成的，OCCl自由基是在266 nm草酰氯光解过程中产生的。
