Liu Tianlin, Elliott Sarah N, Zou Meijun, Vansco Michael F, Sojdak Christopher A, Markus Charles R, Almeida Raybel, Au Kendrew, Sheps Leonid, Osborn David L, Winiberg Frank A F, Percival Carl J, Taatjes Craig A, Caravan Rebecca L, Klippenstein Stephen J, Lester Marsha I
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States.
Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States.
J Am Chem Soc. 2023 Sep 6;145(35):19405-19420. doi: 10.1021/jacs.3c07126. Epub 2023 Aug 25.
Alkene ozonolysis generates short-lived Criegee intermediates that are a significant source of hydroxyl (OH) radicals. This study demonstrates that roaming of the separating OH radicals can yield alternate hydroxycarbonyl products, thereby reducing the OH yield. Specifically, hydroxybutanone has been detected as a stable product arising from roaming in the unimolecular decay of the methyl-ethyl-substituted Criegee intermediate (MECI) under thermal flow cell conditions. The dynamical features of this novel multistage dissociation plus a roaming unimolecular decay process have also been examined with ab initio kinetics calculations. Experimentally, hydroxybutanone isomers are distinguished from the isomeric MECI by their higher ionization threshold and distinctive photoionization spectra. Moreover, the exponential rise of the hydroxybutanone kinetic time profile matches that for the unimolecular decay of MECI. A weaker methyl vinyl ketone (MVK) photoionization signal is also attributed to OH roaming. Complementary multireference electronic structure calculations have been utilized to map the unimolecular decay pathways for MECI, starting with 1,4 H atom transfer from a methyl or methylene group to the terminal oxygen, followed by roaming of the separating OH and butanonyl radicals in the long-range region of the potential. Roaming via reorientation and the addition of OH to the vinyl group of butanonyl is shown to yield hydroxybutanone, and subsequent C-O elongation and H-transfer can lead to MVK. A comprehensive theoretical kinetic analysis has been conducted to evaluate rate constants and branching yields (ca. 10-11%) for thermal unimolecular decay of MECI to conventional and roaming products under laboratory and atmospheric conditions, consistent with the estimated experimental yield (ca. 7%).
烯烃臭氧化反应会产生寿命短暂的克里吉中间体，这些中间体是羟基（OH）自由基的重要来源。本研究表明，分离出的OH自由基的漫游会产生替代的羟基羰基产物，从而降低OH的产率。具体而言，在热流动池条件下，已检测到羟基丁酮是甲基 - 乙基取代的克里吉中间体（MECI）单分子衰变过程中漫游产生的稳定产物。还通过从头算动力学计算研究了这种新型多阶段解离加漫游单分子衰变过程的动力学特征。在实验中，羟基丁酮异构体通过其较高的电离阈值和独特的光电离光谱与异构的MECI区分开来。此外，羟基丁酮动力学时间曲线的指数上升与MECI的单分子衰变曲线相匹配。较弱的甲基乙烯基酮（MVK）光电离信号也归因于OH的漫游。已利用互补的多参考电子结构计算来绘制MECI的单分子衰变途径，首先是从甲基或亚甲基向末端氧进行1,4氢原子转移，然后是分离出的OH和丁酰基自由基在势能的长程区域内漫游。通过重新取向漫游以及OH加成到丁酰基的乙烯基上显示会产生羟基丁酮，随后的C - O伸长和H转移可导致MVK。已进行了全面的理论动力学分析，以评估在实验室和大气条件下MECI热单分子衰变生成常规产物和漫游产物的速率常数和分支产率（约10 - 11%），这与估计的实验产率（约7%）一致。
