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