Unraveling Conformer-Specific Sources of Hydroxyl Radical Production from an Isoprene-Derived Criegee Intermediate by Deuteration
Unraveling Conformer-Specific Sources of Hydroxyl Radical Production from an Isoprene-Derived Criegee Intermediate by Deuteration
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DOI：
10.1021/acs.jpca.0c02867
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发表时间：
2020-06-18
影响因子：
2.9
通讯作者：
Lester, Marsha, I
中科院分区：
文献类型：
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作者：
Hansen, Anne S.;Liu, Ziao;Lester, Marsha, I
异戊二烯，最丰富的挥发性有机化合物排放到地球的对流层后，甲烷的臭氧分解，产生三个不同的Criegee中间体。其中，甲基乙烯基酮氧化物（MVK-氧化物）被预测为来自异戊二烯臭氧分解的大气羟基自由基（OH）的主要来源。先前，Barber等人[J. Am.化学会，2018，140，pp 10866-10880]证明，顺式-MVK-氧化物构象异构体通过从甲基到相邻末端氧原子的1，4-氢（H）转移经历单分子衰变，随后迅速释放OH自由基产物。在这里，我们选择性地氘化MVK-氧化物（d（3）-MVK-氧化物）的甲基，并记录其在乙烯基CH伸缩泛音（2 nu（CH））区域的红外作用光谱。通过激光诱导荧光检测到的OD自由基产物的时间依赖性外观表明，d（3）-MVK-氧化物的单分子衰变通过预期用于顺式构象异构体的类似的1，4-氘（D）原子转移机制进行。实验光谱和时间的结果与计算的红外吸收光谱和单分子衰减率的预测的水稻Ramsperger-Kassel-Marcus（RRKM）理论的syn-d（3）-MVK-氧化物，以及先前的研究syn-MVK-氧化物进行了比较。d（3）-MVK-氧化物的IR作用光谱与MVK-氧化物的IR作用光谱相似，但随着涉及CD拉伸的泛音和组合跃迁向较低频率偏移而表现出显著变化。在2nu（CH）区，d（3）-MVK-氧化物的单分子衰变速率比MVK-氧化物慢40倍。实验上，OD产物的时间分布反映了d（3）-MVK-氧化物的单分子衰减比MVK-氧化物到OH产物的慢以及实验因素。实验和理论都表明，在过渡态势垒附近的能量下，量子力学隧穿在1，4-H/D转移过程中起着非常重要的作用。红外作用光谱的相似性和氘代单分子衰变动力学的变化表明，顺式构象的MVK-氧化物和d（3）-MVK-氧化物的红外作用光谱的主要贡献。
Ozonolysis of isoprene, the most abundant volatile organic compounds emitted into the Earth's troposphere after methane, yields three distinct Criegee intermediates. Among these, methyl vinyl ketone oxide (MVK-oxide) is predicted to be the major source of atmospheric hydroxyl radicals (OH) from isoprene ozonolysis. Previously, Barber et al. [J. Am. Chem. Soc., 2018, 140, pp 10866-10880] demonstrated that syn-MVK-oxide conformers undergo unimolecular decay via 1,4-hydrogen (H) transfer from the methyl group to the adjacent terminal oxygen atom, followed by the prompt release of OH radical products. Here, we selectively deuterate the methyl group of MVK-oxide (d(3)-MVK-oxide) and record its IR action spectrum in the vinyl CH stretch overtone (2 nu(CH)) region. The resultant time-dependent appearance of OD radical products, detected by laser-induced fluorescence, demonstrates that a unimolecular decay of d(3)-MVK-oxide proceeds by an analogous 1,4-deuterium (D) atom transfer mechanism anticipated for syn conformers. The experimental spectral and temporal results are compared with the calculated IR absorption spectrum and unimolecular decay rates predicted by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory for syn-d(3)-MVK-oxide, as well as the prior study on syn-MVK-oxide. The d(3)-MVK-oxide IR action spectrum is similar to that for MVK-oxide, yet exhibits notable changes as the overtone and combination transitions involving CD stretch shift to a lower frequency. The unimolecular decay rate for d(3)-MVK-oxide is predicted to be a factor of 40 times slower than that for MVK-oxide in the 2 nu(CH) region. Experimentally, the temporal profile of the OD products reflects the slower unimolecular decay of d(3)-MVK-oxide compared to that for MVK-oxide to OH products as well as experimental factors. Both experiment and theory demonstrate that quantum mechanical tunneling plays a very important role in the 1,4-H/D-transfer processes at energies in the vicinity of the transition-state barrier. The similarities of the IR action spectra and changes in the unimolecular decay dynamics upon deuteration indicate that syn conformers make the main contribution to the IR action spectra of MVK-oxide and d(3)-MVK-oxide.