Auto-Oxidation of a Volatile Silicon Compound: A Theoretical Study of the Atmospheric Chemistry of Tetramethylsilane

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.

Atmospheric Degradation of Cyclic Volatile Methyl Siloxanes: Radical Chemistry and Oxidation Products

Cyclic volatile methyl siloxanes (cVMS) are anthropogenic chemicals that have come under scrutiny due to their widespread use and environmental persistence. Significant data on environmental concentrations and persistence of these chemicals exists, but their oxidation mechanism is poorly understood, preventing a comprehensive understanding of the environmental fate and impact of cVMS. We performed experiments in an environmental chamber to characterize the first-generation oxidation products of hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5) under different peroxy radical fates (unimolecular reaction or bimolecular reaction with either NO or HO₂) that approximate a range of atmospheric compositions. While the identity of the oxidation products from D3 changed as a function of the peroxy radical fate, the identity and yield of D4 and D5 oxidation products remained largely constant. We compare our results against the output from a kinetic model of cVMS oxidation chemistry. The reaction mechanism used in the model is developed using a combination of previously proposed cVMS oxidation reactions and standard atmospheric oxidation radical chemistry. We find that the model is unable to reproduce our measurements, particularly in the case of D4 and D5. The products that are poorly represented in the model help to identify possible branching points in the mechanism, which require further investigation. Additionally, we estimated the physical properties of the cVMS oxidation products using structure-activity relationships and found that they should not be significantly partitioned to organic or aqueous aerosol. The results suggest that cVMS first-generation oxidation products are also long-lived in the atmosphere and that environmental monitoring of these compounds is necessary to understand the environmental chemistry and loading of cVMS.