OH Roaming and Beyond in the Unimolecular Decay of the Methyl-Ethyl-Substituted Criegee Intermediate: Observations and Predictions
OH Roaming and Beyond in the Unimolecular Decay of the Methyl-Ethyl-Substituted Criegee Intermediate: Observations and Predictions
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甲基乙基取代的 Criegee 中间体的单分子衰变中的 OH 漫游和超越：观察和预测
DOI：
10.1021/jacs.3c07126
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发表时间：
2023
影响因子：
15
通讯作者：
Osborn, David L.
中科院分区：
文献类型：
--
作者：
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.
烯烃臭氧分解产生短寿命的Criegee中间体，其是羟基（OH）自由基的重要来源。这项研究表明，游离OH自由基的漫游可以产生交替的羟基羰基产物，从而降低OH产率。具体而言，羟基丁酮已被检测到作为一种稳定的产品所产生的漫游在单分子衰变的甲基-乙基取代的Criegee中间体（MECI）在热流池条件下。用从头算动力学计算研究了这种新型的多级解离加漫游单分子衰变过程的动力学特征。实验上，羟基丁酮异构体区别于同分异构体的MECI，其较高的电离阈值和独特的光电离光谱。此外，羟基丁酮动力学时间曲线的指数上升与MECI的单分子衰变相匹配。一个较弱的甲基乙烯基酮（MVK）的光电离信号也归因于OH漫游。互补的多参考电子结构计算已被用来映射MECI的单分子衰变途径，从1，4 H原子从甲基或亚甲基转移到末端氧，然后漫游的分离OH和丁酰基自由基在远程区域的潜力。通过重定向和OH加成到丁酮基的乙烯基上的漫游被示出产生羟基丁酮，并且随后的C-O伸长和H-转移可以导致MVK。已经进行了全面的理论动力学分析，以评估速率常数和支化产率（约。10-11%）的热单分子衰变的MECI的常规和漫游产品在实验室和大气条件下，与估计的实验产量（约。7%）。
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%).