Kinetics and Mechanism of the Oxidation of Cyclic Methylsiloxanes by Hydroxyl Radical in the Gas Phase: An Experimental and Theoretical Study

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• 作者：Ruiyang Xiao ; Ian Zammit ; Zongsu Wei ; Wei-Ping Hu ; Matthew MacLeod ; Richard Spinney
• 刊名：Environmental Science & Technology
• 出版年：2015
• 出版时间：November 17, 2015
• 年：2015
• 卷：49
• 期：22
• 页码：13322-13330
• 全文大小：454K
• ISSN：1520-5851

文摘

The ubiquitous presence of cyclic volatile methylsiloxanes (cVMS) in the global atmosphere has recently raised environmental concern. In order to assess the persistence and long-range transport potential of cVMS, their second-order rate constants (k) for reactions with hydroxyl radical (^{鈥?/sup>OH) in the gas phase are needed. We experimentally and theoretically investigated the kinetics and mechanism of 鈥?/sup>OH oxidation of a series of cVMS, hexamethylcyclotrisiloxane (D₃), octamethycyclotetrasiloxane (D₄), and decamethycyclopentasiloxane (D₅). Experimentally, we measured k values for D₃, D₄, and D₅ with 鈥?/sup>OH in a gas-phase reaction chamber. The Arrhenius activation energies for these reactions in the temperature range from 313 to 353 K were small (鈭?.92 to 0.79 kcal路mol鈥?), indicating a weak temperature dependence. We also calculated the thermodynamic and kinetic behaviors for reactions at the M06-2X/6-311++G**//M06-2X/6-31+G** level of theory over a wider temperature range of 238鈥?58 K that encompasses temperatures in the troposphere. The calculated Arrhenius activation energies range from 鈭?.71 to 鈭?.64 kcal路mol鈥?, also exhibiting weak temperature dependence. The measured k values were approximately an order of magnitude higher than the theoretical values but have the same trend with increasing size of the siloxane ring. The calculated energy barriers for H-atom abstraction at different positions were similar, which provides theoretical support for extrapolating k for other cyclic siloxanes from the number of abstractable hydrogens.}