Volatile Methyl Siloxane Atmospheric Oxidation Mechanism from a Theoretical Perspective─How is the Siloxanol Formed?
Volatile Methyl Siloxane Atmospheric Oxidation Mechanism from a Theoretical Perspective─How is the Siloxanol Formed?
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理论视角下的挥发性甲基硅氧烷大气氧化机理——硅氧烷醇是如何形成的？
DOI：
10.1021/acs.jpca.3c06287
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发表时间：
2023
期刊：
影响因子：
--
通讯作者：
Browne, Eleanor C.
中科院分区：
文献类型：
--
作者：
Alton, Mitchell W.;Johnson, Virginia L.;Sharma, Sandeep;Browne, Eleanor C.
尽管对环状挥发性甲基硅氧烷（VMS）在大气中的归宿进行了多次研究，但对这些纯人为的高产量化合物的氧化化学知之甚少。这导致了氧化产物的环境影响和归宿的不确定性。根据实验室的测量，VMS的主要氧化产物是藜芦醇（− CH 3被−OH取代）;然而，迄今为止提出的任何机制都不能令人满意地解释其形成。受我们先前对VMS氧化产物的实验观察的启发，我们使用理论量子化学计算来（1）探索由甲硅烷氧基自由基与气相水反应形成十八烷醇的先前未考虑的反应途径，（2）研究六甲基环三硅氧烷（D3）和八甲基环四硅氧烷（D4）氧化中自由基中间体的反应速率的差异，（3）试图解释实验观察到的产物。我们的研究结果表明，虽然所提出的反应的甲硅烷氧基自由基与水形成的藜芦醇可以发生，它是太慢，竞争与其他单分子反应，因此不能解释所观察到的藜芦醇形成。我们还发现，起始的D3过氧自由基（RO 2·）与HO 2·之间的反应比以前预期的要慢（D3为3 × 10- 13 cm 3 molecule-1 s-1，D4为2 × 10- 11 cm 3 molecule-1 s-1，而一般的反应速率为10 × 10- 11 cm 3 molecule-1 s-1）.最后，我们比较了RO 2·在各种条件下的预期命运，发现与NO的反应（假设一般RO 2·+ NO双分子速率常数为9 × 10- 12 cm 3 molecule-1 s-1）可能是城市条件下的主要命运，而异构化在清洁环境中可能很重要。
Despite several investigations on the atmospheric fate of cyclic volatile methyl siloxanes (VMS), the oxidation chemistry of these purely anthropogenic, high production volume compounds is poorly understood. This led to uncertainties in the environmental impact and fate of the oxidation products. According to laboratory measurements, the main VMS oxidation product is the siloxanol (a −CH3replaced with an −OH); however, none of the mechanisms proposed to date satisfactorily explain its formation. Motivated by our previous experimental observations of VMS oxidation products, we use theoretical quantum chemical calculations to (1) explore a previously unconsidered reaction pathway to form the siloxanol from a reaction of a siloxy radical with gas-phase water, (2) investigate differences in reaction rates of radical intermediates in hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4) oxidation, and (3) attempt to explain the experimentally observed products. Our results suggest that while the proposed reaction of the siloxy radical with water to form the siloxanol can occur, it is too slow to compete with other unimolecular reactions and thus cannot explain the observed siloxanol formation. We also find that the reaction between the initial D3 peroxy radical (RO2•) with HO2•is slower than previously anticipated (calculated as 3 × 10–13cm3molecule–1s–1for D3 and 2 × 10–11cm3molecule–1s–1for D4 compared to the general rate of ∼1 × 10–11cm3molecule–1s–1). Finally, we compare the anticipated fates of the RO2•under a variety of conditions and find that a reaction with NO (assuming a general RO2•+ NO bimolecular rate constant of 9 × 10–12cm3molecule–1s–1) will likely be the dominant fate in urban conditions, while isomerization can be important in cleaner environments.
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