Alkyl Nitrites as a Model System to Probe the Autoxidation of Peroxy Radicals

Abstract:

The formation and fate of peroxy radicals (RO₂) plays a key role in constraining the atmospheric oxidative capacity. RO₂ H-shift chemistry, or autoxidation, has been implicated in sustaining the radical pool as the removal of species requires fewer oxidative steps. Recent studies have suggested that autoxidation plays a role in the chemistry of isoprene and terpenes, in many cases forming highly oxidized multifunctional compounds with low vapor pressures. A fundamental understanding of the mechanism and products is needed for models to accurately characterize the effect of autoxidation. A laboratory kinetics study of autoxidation in a model system of alkyl nitrites is presented. The nitrites (1-hexylnitrite, 2-hexylnitrite, and 2-methyl-2-hexylnitrite) possess varying structural properties which influence the H-shift kinetics. The compounds were photolyzed in an environmental chamber to produce RO₂ following an alkoxy H-shift and CF₃O^- chemical ionization mass spectrometry (CIMS) was employed in the detection of products. The RO₂ isomerization coefficients were derived at 296 and 313 K, and relied on the relative production yields of the alkyl nitrate and isomerization product. Our results support a quantitative ranking of these substituent effects. This knowledge will aid both laboratory and simulation work in evaluating the importance of autoxidation in relevant atmospheric systems.