Volatile Methyl Siloxane Atmospheric Oxidation Mechanism from a Theoretical Perspective─How is the Siloxanol Formed?

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 -CH₃ replaced 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^-13 cm³ molecule^-1 s^-1 for D3 and 2 × 10^-11 cm³ molecule^-1 s^-1 for D4 compared to the general rate of ∼1 × 10^-11 cm³ molecule^-1 s^-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^-12 cm³ molecule^-1 s^-1) will likely be the dominant fate in urban conditions, while isomerization can be important in cleaner environments.