title: Kinetics and Mechanism of the Oxidation of Cyclic Methylsiloxanes by Hydroxyl Radical in the Gas Phase: An Experimental and Theoretical Study.
abstract: 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) in the gas phase are needed. We experimentally and theoretically investigated the kinetics and mechanism of (•)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 (•)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 (-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.
authors.affiliation: Institute of Environmental Engineering, School of Metallurgy and Environment, Central South University , Changsha, Hunan 410083, China.|Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution , Changsha, Hunan 410083, China.
authors.forename: Ruiyang
authors.initials: R
authors.lastname: Xiao
authors.affiliation: Department of Environmental Science and Analytical Chemistry, Stockholm University , Svante Arrhenius väg 8, Stockholm SE-11418, Sweden.
authors.forename: Ian
authors.initials: I
authors.lastname: Zammit
authors.forename: Zongsu
authors.initials: Z
authors.lastname: Wei
authors.affiliation: Department of Chemistry and Biochemistry, National Chung Cheng University , Minxiong, Chia-Yi 62102, Taiwan.
authors.forename: Wei-Ping
authors.initials: WP
authors.lastname: Hu
authors.affiliation: Department of Environmental Science and Analytical Chemistry, Stockholm University , Svante Arrhenius väg 8, Stockholm SE-11418, Sweden.
authors.forename: Matthew
authors.initials: M
authors.lastname: MacLeod
authors.forename: Richard
authors.initials: R
authors.lastname: Spinney
mesh_terms: D000393:Air Pollutants / Q000737:chemistry*; D001272:Atmosphere; D005740:Gases / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry; D017665:Hydroxyl Radical / Q000737:chemistry; D007700:Kinetics; D008956:Models, Chemical; D010084:Oxidation-Reduction; D012833:Siloxanes / Q000737:chemistry*; D013696:Temperature; D013816:Thermodynamics
chemical_list: D000393:Air Pollutants; D005740:Gases; D012833:Siloxanes; D017665:Hydroxyl Radical; D006859:Hydrogen; C024035:hexamethylcyclotrisiloxane