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

Abstract

Volatile silicon compounds (VOSiCs) are air pollutants present in both indoor and outdoor environments. Here, tetramethylsilane (TMS) is selected as a model to study the photochemical oxidation mechanisms for VOSiCs using ab initio and RRKM theory/master equation kinetic modeling. Under tropospheric conditions, the radical (CH3)3SiCH2• reacts with O2 to produce a stabilized peroxyl radical, which is expected to ultimately yield the alkoxyl radical (CH3)3SiCH2O•. At combustion-relevant temperatures, however, a well-skipping reaction to (CH3)3SiO• + HCHO dominates. Importantly, the (CH3)3SiCH2O• radical is predicted to rearrange to (CH3)3SiOCH2• with a very low reaction barrier, enabling an auto-oxidation process involving addition of a second O2. Subsequent oxidation reaction mechanisms of (CH3)3SiOCH2• have been developed, with the major product predicted to be the trimethylsilyl formate (CH3)3SiOCHO, an experimentally observed TMS oxidation product. The production of substantially oxygenated compounds following a single radical initiation reaction has implications for the ability of VOSiCs to contribute to ozone and particle formation in both outdoor and indoor environments.