Products of the Gas-Phase Reactions of a Series of Methyl-Substituted Ethenes with the OH Radical

Alkenes are a significant component of ambient air in urban areas, and in the atmosphere they react with OH radicals, NO3 radicals, and O3, with the OH radical reaction often dominating as the tropospheric removal process. Products of the gas-phase reactions of the OH radical with 2-methylpropene, cis-2-butene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene in the presence of NO have been investigated at room temperature and 740 Torr total pressure of air by gas chromatography with flame ionization detection, in situ Fourier transform infrared absorption spectroscopy, and in situ atmospheric pressure ionization tandem mass spectrometry (API-MS). For all four alkenes, hydroxynitrate formation from the β-hydroxyalkyl peroxy radical plus NO reaction accounted for ∼10% of the overall products observed, with decomposition of the intermediate β-hydroxyalkoxy radical dominating over the alternative reaction with O2 (isomerization via a six-member transition state is not possible for any of the intermediate β-hydroxyalkoxy radicals formed in these reactions) and leading to the formation of the carbonyls expected from >CC< bond cleavage in ∼90% yield each. These data are consistent with estimates of the rates of the various reactions of β-hydroxyalkoxy radicals under atmospheric conditions. API-MS analyses of the OH radical-initiated reactions of propene, 2-methylpropene, cis- and trans-2-butene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene in the absence of NO indicated the formation of the hydroxyhydroperoxides, dihydroxyalkanes, and hydroxycarbonyls expected from the reactions of intermediate β-hydroxyalkyl peroxy radicals with HO2 and organic peroxy radicals, in addition to the carbonyl compounds formed from decomposition of the β-hydroxyalkoxy radicals also formed from the peroxy plus peroxy radical reactions.