Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production
Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production
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DOI：
10.1021/jacs.8b06010
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发表时间：
2018-08-29
影响因子：
15
通讯作者：
Lester, Marsha I.
中科院分区：
文献类型：
--
作者：
Barber, Victoria P.;Pandit, Shubhrangshu;Lester, Marsha I.
异戊二烯是大气中最丰富的挥发性有机化合物之一，臭氧与异戊二烯的反应会产生三种不同的羰基氧化物 (RR'COO)，称为 Criegee 中间体：氧化甲醛 (CH2OO)、氧化甲基乙烯基酮 (MVK-OO) 和氧化异丁烯醛 (MACR-OO)。 Criegee 中间体的取代基 (R,R' = H, CH3, CH=CH2) 的性质和构象控制着它们随后在大气中的化学反应。特别是，MVK-OO 的单分子衰变预计是异戊二烯臭氧分解中羟基自由基 (OH) 的主要来源。本研究报告了通过光解产生的、共振稳定的单碘烯自由基与 O-2 反应，初步实验室合成并直接检测 MVK-OO。 MVK-OO 利用红外 (IR) 作用光谱进行表征，其中 MVK-OO 的红外激活与两个量子的 CH 拉伸在约 100°C 处。 6000 cm(-1) 与所得 OH 产物的紫外检测相结合。通过将实验观察到的红外光谱特征与理论预测的红外吸收光谱进行比较来识别 MVK-OO。对于 syn-MVK-OO，OH 产物的出现速率与使用隧道效应统计理论预测的单分子衰减速率一致。这验证了氢原子转移机制和计算出的导致 OH 产物的过渡态势垒 (18.0 kcal mol(-1))。理论计算揭示了分离自由基片段之间的额外漫游路径，从而产生其他产物。主方程模型得出 syn-MVK-OO 的热单分子衰减率为 33 s(-1) (298 K, 1 atm)。对于抗 MVK-OO，对几种单分子衰变途径的理论探索预测，异构化为间二氧杂环戊烯是最有可能获得产品的第一步。
The reaction of ozone with isoprene, one of the most abundant volatile organic compounds in the atmosphere, produces three distinct carbonyl oxide species (RR'COO) known as Criegee intermediates: formaldehyde oxide (CH2OO), methyl vinyl ketone oxide (MVK-OO), and methacrolein oxide (MACR-OO). The nature of the substituents (R,R' = H, CH3, CH=CH2) and conformations of the Criegee intermediates control their subsequent chemistry in the atmosphere. In particular, unimolecular decay of MVK-OO is predicted to be the major source of hydroxyl radicals (OH) in isoprene ozonolysis. This study reports the initial laboratory synthesis and direct detection of MVK-OO through reaction of a photolytically generated, resonance-stabilized monoiodoalkene radical with O-2. MVK-OO is characterized utilizing infrared (IR) action spectroscopy, in which IR activation of MVK-OO with two quanta of CH stretch at ca. 6000 cm(-1) is coupled with ultraviolet detection of the resultant OH products. MVK-OO is identified by comparison of the experimentally observed IR spectral features with theoretically predicted IR absorption spectra. For syn-MVK-OO, the rate of appearance of OH products agrees with the unimolecular decay rate predicted using statistical theory with tunneling. This validates the hydrogen atom transfer mechanism and computed transition-state barrier (18.0 kcal mol(-1)) leading to OH products. Theoretical calculations reveal an additional roaming pathway between the separating radical fragments, which results in other products. Master equation modeling yields a thermal unimolecular decay rate for syn-MVK-OO of 33 s(-1) (298 K, 1 atm). For anti-MVK-OO, theoretical exploration of several unimolecular decay pathways predicts that isomerization to dioxole is the most likely initial step to products.