Products and Mechanism of the Reaction of 1-Pentadecene with NO₃ Radicals and the Effect of a −ONO₂ Group on Alkoxy Radical Decomposition

Abstract

The linear C15 alkene, 1-pentadecene, was reacted with NO3 radicals in a Teflon environmental chamber and yields of secondary organic aerosol (SOA) and particulate β-hydroxynitrates, β-carbonylnitrates, and organic peroxides (β-nitrooxyhydroperoxides + dinitrooxyperoxides) were quantified using a variety of methods. Reaction occurs almost solely by addition of NO3 to the C═C double bond and measured yields of β-hydroxynitrate isomers indicate that 92% of addition occurs at the terminal carbon. Molar yields of reaction products determined from measurements, a proposed reaction mechanism, and mass-balance considerations were 0.065 for β-hydroxynitrates (0.060 and 0.005 for 1-nitrooxy-2-hydroxypentadecane and 1-hydroxy-2-nitrooxypentadecane isomers), 0.102 for β-carbonylnitrates, 0.017 for organic peroxides, 0.232 for β-nitrooxyalkoxy radical isomerization products, and 0.584 for tetradecanal and formaldehyde, the volatile C14 and C1 products of β-nitrooxyalkoxy radical decomposition. Branching ratios for decomposition and isomerization of β-nitrooxyalkoxy radicals were 0.716 and 0.284 and should be similar for other linear 1-alkenes ≥ C6 whose alkyl chains are long enough to allow for isomerization to occur. These branching ratios have not been measured previously, and they differ significantly from those estimated using structure-activity relationships, which predict >99% isomerization. It appears that the presence of a -ONO2 group adjacent to an alkoxy radical site greatly enhances the rate of decomposition relative to isomerization, which is otherwise negligible, and that the effect is similar to that of a -OH group. The results provide insight into the effects of molecular structure on mechanisms of oxidation of volatile organic compounds and should be useful for improving structure-activity relationships that are widely used to predict the fate of these compounds in the atmosphere and for modeling SOA formation and aging.

Experimental and theoretical study on the impact of a nitrate group on the chemistry of alkoxy radicals

“…The current set of experimental and theoretical data is in disagreement with the experimental data by Yeh et al, 18 who concluded that the decomposition and isomerisation of aliphatic nitrate-RO have comparable rates, and that the alkoxy radicals SARs by Vereecken and Peeters 13,59 thus strongly underestimate the rate of decomposition relative to isomerisation. Our current set of high-level theoretical results still finds that decomposition is slower, and that isomerisation dominates over decomposition, in agreement with the original SARs.…”

“…They propose that the decomposition SAR by Vereecken and Peeters 13 is incorrect, overestimating the barrier heights for b-ONO 2 -substituted alkoxy radicals by B3.7 kcal mol À1 ; this reduction in barrier height would allow decomposition to be competitive against H-migration as predicted by the SAR by Vereecken and Peeters. 59 The current theoretical results remain in clear disagreement with the result of Yeh et al 18 The higher-level calculations presented in this work support the barrier heights for decomposition predicted by the SAR by Vereecken and Peeters, 13 and even find that they should be slightly higher than the predictions by this SAR for 1-ONO 2 -2-pentadecoxy. While we did not examine pentadecene-derived alkoxy radicals directly due to the computational cost, the H-migration rates in 1-pentene and 2-hexenederived nitrate-RO remain in agreement with the H-migration SAR by Vereecken and Peeters, 59 dominating over the decomposition by several orders of magnitude.…”

“…13 This SAR is based on theoretical methodologies that are considered less accurate by nowadays standards yet matches the available experimental data well, but the SAR does not account fully for interactions between multiple substituents on the carbons of the decomposing C-C bond. Based on an indirect experimental product study for 1-pentadecene + NO 3 , Yeh et al 18 suggested that the rate of decomposition of NO 3 -substituted alkoxy radicals (called nitrate-RO hereafter) is underestimated by that SAR by several orders of magnitude. As decomposition of RO radicals competes against H-migration or reaction with O 2 , [19][20][21][22][23] the rate of decomposition has an important impact on the atmospheric fate of nitrated intermediates, and hence the products formed and the rate of oxidative removal of VOCs from the atmosphere.…”

“…To our knowledge, no direct measurements of nitrate-RO or epoxy-RO decomposition or isomerisation reactions are available in the literature. Yeh et al 18 describe an experiment on NO 3 -initiated oxidation of 1-pentadecene, where the products are collected on filters and analyzed. Based on the observed products, they conclude that isomerisation and decomposition reactions of nitrate-RO radicals are comparable in rate, while the expectation from SARs is that H-migration is significantly faster.…”

“…To illustrate the impact of the faster decomposition rate proposed by Yeh et al, we performed model simulations replacing our theory-predicted decomposition rates with values based on their findings. In this sensitivity study, we set decomposition for nitrate-RO from 1-pentene and 2-hexene to a rate 2.5 times faster than the theory-predicted isomerisation rate, mimicking the 0.397 ratio of isomerisation to decomposition by Yeh et al For propene, we calculated decomposition rate coefficients from the SAR by Vereecken and Peeters 13 with an additional 3.7 kcal mol À1 reduction in barrier height as proposed by Yeh et al 18 As this strong increase in rate also changes the competition of decomposition against the RO + O 2 reaction and hence the HO 2 balance in the chamber, model runs were performed with the HO 2 constrained to the measurements, to avoid secondary effects. The results are depicted in Fig.…”