Doner Anna C, Zádor Judit, Rotavera Brandon
College of Engineering, Department of Chemistry, University of Georgia, Athens, Georgia 30602, USA.
Combustion Research Facility, Sandia National Laboratories, 7011 East Avenue, Livermore, CA 94550, USA.
Faraday Discuss. 2022 Oct 21;238(0):295-319. doi: 10.1039/d2fd00029f.
2,4,dimethyloxetane is an important cyclic ether intermediate that is produced from hydroperoxyalkyl (QOOH) radicals in the low-temperature combustion of -pentane. However, the reaction mechanisms and rates of consumption pathways remain unclear. In the present work, the pressure- and temperature-dependent kinetics of seven cyclic ether peroxy radicals, which stem from 2,4,dimethyloxetane H-abstraction and O addition, were determined. The automated kinetic workflow code, KinBot, was used to model the complexity of the chemistry in a stereochemically resolved manner and solve the resulting master equations from 300-1000 K and from 0.01-100 atm. The main conclusions from the calculations include (i) diastereomeric cyclic ether peroxy radicals show significantly different reactivities, (ii) the stereochemistry of the peroxy radical determines which QOOH isomerization steps are possible, (iii) conventional QOOH decomposition pathways, such as cyclic ether formation and HO elimination, compete with ring-opening reactions, which primarily produce OH radicals, the outcome of which is sensitive to stereochemistry. Ring-opening reactions lead to unique products, such as unsaturated, acyclic peroxy radicals, that form direct connections with species present in other chemical kinetics mechanisms through "cross-over" reactions that may complicate the interpretation of experimental results from combustion of n-pentane and, by extension, other alkanes. For example, one cross-over reaction involving 1-hydroperoxy-4-pentanone-2-yl produces 2-(hydroperoxymethyl)-3-butanone-1-yl, which is an iso-pentane-derived ketohydroperoxide (KHP). At atmospheric pressure, the rate of chemical reactions of all seven peroxy radicals compete with that of collisional stabilization, resulting in well-skipping reactions. However, at 100 atm, only one out of seven peroxy radicals undergoes significant well-skipping reactions. The rates produced from the master equation calculations provide the first foundation for the development of detailed sub-mechanisms for cyclic ether intermediates. In addition, analysis of the complex reaction mechanisms of 2,4-dimethyloxetane-derived peroxy radicals provides insights into the effects of stereoisomers on reaction pathways and product yields.
2,4 - 二甲基氧杂环丁烷是一种重要的环状醚中间体，它由正戊烷低温燃烧过程中的氢过氧烷基（QOOH）自由基产生。然而，其反应机理和消耗途径的速率仍不明确。在本工作中，测定了源自2,4 - 二甲基氧杂环丁烷氢原子提取和氧加成的七种环状醚过氧自由基的压力和温度依赖动力学。使用自动动力学工作流程代码KinBot以立体化学解析的方式对化学复杂性进行建模，并求解300 - 1000 K和0.01 - 100 atm下产生的主方程。计算得出的主要结论包括：（i）非对映异构的环状醚过氧自由基表现出显著不同的反应活性；（ii）过氧自由基的立体化学决定了哪些QOOH异构化步骤是可能的；（iii）传统的QOOH分解途径，如环状醚形成和羟基消除，与开环反应竞争，开环反应主要产生羟基自由基，其结果对立体化学敏感。开环反应导致独特的产物，如不饱和的、无环过氧自由基，它们通过“交叉”反应与其他化学动力学机制中存在的物种形成直接联系，这可能会使正戊烷燃烧以及由此推广到其他烷烃的实验结果解释变得复杂。例如，一个涉及1 - 氢过氧 - 4 - 戊酮 - 2 - 基的交叉反应产生2 - （氢过氧甲基） - 3 - 丁酮 - 1 - 基，它是一种异戊烷衍生的酮氢过氧化物（KHP）。在大气压下，所有七种过氧自由基的化学反应速率与碰撞稳定化速率竞争，导致跳阱反应。然而，在100 atm下，七种过氧自由基中只有一种发生显著的跳阱反应。主方程计算得出的速率为环状醚中间体详细子机制的开发提供了首个基础。此外，对2,4 - 二甲基氧杂环丁烷衍生的过氧自由基复杂反应机制的分析为立体异构体对反应途径和产物产率的影响提供了见解。
