Combustion Research Facility , Sandia National Laboratories , Livermore , California 94551 , United States.
Physikalisch-Technische Bundesanstalt , Bundesallee 100 , 38116 Braunschweig , Germany.
J Phys Chem A. 2019 Sep 26;123(38):8274-8284. doi: 10.1021/acs.jpca.9b07017. Epub 2019 Sep 17.
Keto-hydroperoxides (KHPs) are reactive, partially oxidized intermediates that play a central role in chain-branching reactions during the gas-phase low-temperature oxidation of hydrocarbons and oxygenated species. Although multiple isomeric forms of the KHP intermediate are possible in complex oxidation environments when multiple reactant radicals exist that contain nonequivalent O addition sites, isomer-resolved data of KHPs have not been reported. In this work, we provide partially isomer-resolved detection and quantification of the KHPs that form during the low-temperature oxidation of tetrahydrofuran (THF, -O-CHCHCHCH-). We describe how these short-lived KHPs were detected, identified, and quantified using integrated experimental and theoretical approaches. The experimental approaches were based on direct molecular-beam sampling from a jet-stirred reactor operated at near-atmospheric pressure and at temperatures between 500 and 700 K, followed by mass spectrometry with single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation, and the identification of fragmentation patterns. The interpretation of the experiments was guided by theoretical calculations of ionization thresholds, fragment appearance energies, and photoionization cross sections. On the basis of the experimentally observed and theoretically calculated ionization and fragment appearance energies, KHP isomers could be distinguished as originating from H-abstraction reactions from either the α-C adjacent to the O atom or the β-C atoms. Temperature-dependent concentration profiles of the partially resolved isomeric KHP intermediates were determined in the range of 500-700 K, and the results indicate that the observed KHP isomers are formed overwhelmingly (∼99%) from the α-C THF radical. Comparisons of the partially isomer-resolved quantification of the KHPs to up-to-date kinetic modeling results reveal new opportunities for the development of a next-generation THF oxidation mechanism.
酮过氧化物（KHP）是反应性的、部分氧化的中间体，在烃类和含氧物种的气相低温氧化过程中，它们在支链反应中起着核心作用。虽然当存在多个含有不等价 O 加成位点的反应物自由基时，复杂氧化环境中可能存在多种 KHP 中间体的异构形式，但尚未报道 KHP 的异构分辨数据。在这项工作中，我们提供了在四氢呋喃（THF，-O-CHCHCHCH-）低温氧化过程中形成的 KHP 的部分异构分辨检测和定量。我们描述了如何使用集成的实验和理论方法来检测、识别和定量这些短寿命的 KHP。实验方法基于直接从在接近大气压和 500-700 K 之间的温度下操作的喷射搅拌反应器中进行分子束采样，然后使用单光子电离通过可调谐同步辐射产生的真空紫外辐射进行质谱分析，并识别碎片模式。实验的解释是通过理论计算来指导的，包括电离阈值、碎片出现能和光离子化截面。根据实验观察到的和理论计算出的电离和碎片出现能，可以区分 KHP 异构体，它们源自与氧原子相邻的α-C 或β-C 原子上的 H 抽提反应。在 500-700 K 的范围内确定了部分分辨异构 KHP 中间体的浓度分布，结果表明，观察到的 KHP 异构体主要（约 99%）来自于α-C THF 自由基。将部分分辨的 KHP 定量与最新的动力学模型结果进行比较，为开发下一代 THF 氧化机制提供了新的机会。
