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Kinetics and Mechanism of the Oxidation of Cyclic Methylsiloxanes by Hydroxyl Radical in the Gas Phase: An Experimental and Theoretical Study
Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
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Number of Authors: 6
2015 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 49, no 22, 13322-13330 p.Article in journal (Refereed) Published
Abstract [en]

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 ((OH)-O-center dot) in the gas phase are needed. We experimentally and theoretically investigated the kinetics and mechanism of (OH)-O-center dot oxidation of a series of cVMS, hexamethylcyclotrisiloxane (D-3), octamethycyclotetrasiloxane (D-4), and decamethycyclopentasiloxane (D-5). Experimentally, we measured k values for D-3, D-4, and D-s -with (OH)-O-center dot in a gas-phase reaction chamber. The Arrhenius activation energies for these reactions in the temperature range from 313 to 353 K were small (-2.92 to 0.79 kcal.mol(-1)), 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 238-358 K that encompasses temperatures in the troposphere. The calculated Arrhenius activation energies range from -2.71 to -1.64 kcal-mol(-1), 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.

Place, publisher, year, edition, pages
2015. Vol. 49, no 22, 13322-13330 p.
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Environmental Engineering Earth and Related Environmental Sciences
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URN: urn:nbn:se:su:diva-124756DOI: 10.1021/acs.est.5b03744ISI: 000365151200027PubMedID: 26477990OAI: oai:DiVA.org:su-124756DiVA: diva2:892182
Available from: 2016-01-08 Created: 2016-01-04 Last updated: 2016-01-08Bibliographically approved

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Zammit, IanMacLeod, Matthew
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