Detailed Investigation of 2,3-Dimethyl-2-butene Ozonolysis-Derived Hydroxyl, Peroxy, and Alkoxy Radical Chemistry

This work investigates the chemistry of peroxy and alkoxy radicals derived from 2,3-dimethyl-2-butene [tetramethylethylene (TME)] ozonolysis. We utilize a combination of computational chemistry and flow reactor chemical ionization mass spectrometry (CIMS) at different temperatures for this study. We particularly focus on the decomposition reactions of alkoxy radicals derived from acetyl peroxy and acetonyl peroxy radicals adding to the TME double bond. The results demonstrate that a great variety of accretion products are formed on the ∼3 s residence time scale of the experiment. The computational chemistry supports the experimental results by inferring assignment of molecular structures to observed mass signals and by explaining the relative concentration of the most abundant peroxides at the different temperatures. Additionally, the computational results suggest that several different unimolecular decomposition pathways are rapid enough to happen on the time scale of the experiment for an acetyl peroxy (APR) + TME-derived alkoxy radical. However, the experimental results tentatively suggest that these alkoxy radicals undergo a methyl β-scission reaction at a competitive rate, despite a more substituted and thus seemingly more favorable β-scission being available. We use computational chemistry to investigate and calculate rate coefficients for the different possible unimolecular decomposition pathways for the APR + TME-derived alkoxy radical and find that the methyl β-scission should be out-competed by the more substituted β-scission, in apparent disagreement with the experimental results. This work is relevant to experimental design, as TME ozonolysis is typically employed as a light-free source of OH radicals in gas phase kinetic experiments. Our findings do not discredit TME ozonolysis as a useful OH radical source; it is important to be aware of possible interferences from TME-derived peroxy and alkoxy radicals if high reactant concentrations are used. Furthermore, the work has principal importance to the investigation of oxidative atmospheric organic chemistry. The radicals investigated here follow a priori unexpected reaction pathways, which demonstrate that these pathways should be considered for other atmospherically relevant organics, where the radicals explored here can serve as a model for future investigations into similar radicals.