Quantitative structure-reactivity relationships of hydroxyl radical rate constants for linear and cyclic volatile methylsiloxanes

An accurate understanding of the fate of volatile methylsiloxanes (VMS) in air is crucial for determining their persistence and concentrations in the environment. Although oxidation by atmospheric hydroxyl radicals (•OH) is considered as a major degradation mechanism for airborne VMS, the existing bimolecular rate constants with •OH measured and modeled for any given VMS compound varied greatly, depending on the approaches used to generate the data. The objectives of the present study were to measure •OH reaction rate constants for 4 cyclic and 4 linear VMS based on a relative rate method using a newly designed atmospheric chamber and to establish structure-reactivity relationships for the kinetics. In the past, the reaction rate constants for VMS were generally recognized to increase with the number of the methyl groups per molecule, the only differential factor in the existing models. However, the new measurements indicated that molecular structure should also be considered in the prediction of the reaction rates. Better empirical models were developed by simple and multiple linear regressions of the measured values from the present study and the literature. A high correlation existed for the reaction rates with the number of the methyl group attached at 2 distinct siloxane structures (i.e., linear and cyclic VMS). Even better correlations were obtained with one or 2 molecular descriptors that are directly related to the size of VMS, which, in turn, not only depend on the number of methyl groups, but the linear/cyclic structures as well for permethylsiloxanes. Environ Toxicol Chem 2017;36:3240-3245. © 2017 SETAC.