Double Bonds are Key to Fast Unimolecular Reactivity in First Generation Monoterpene Hydroxy Peroxy Radicals

Monoterpenes are a group of volatile organic compounds that are emitted to the atmosphere in large amounts by natural sources. Some monoterpenes such as limonene and Δ³-carene are also widely used as additives in detergents and perfumes, and thus have a potential impact on indoor air quality and human health.

The volatile organic compounds like monoterpenes may undergo a series of autoxidation processes in the atmosphere to form highly oxygenated compounds, which have been linked to the formation of secondary organic aerosols. For this process to occur, the unimolecular reactions of the peroxy radicals formed during oxidation must have rate coefficients comparable to or greater than those of the competing bimolecular reactions with HO₂, NO or other RO₂ radicals.

We studied the hydrogen shift (H-shift) and the cyclization reactions of all 45 hydroxy peroxy radicals formed by hydroxyl radical (OH) and O₂ addition to six monoterpenes (α-pinene, β-pinene, Δ³-carene, camphene, limonene and terpinolene). The reaction rate coefficients of the possible unimolecular reaction were initially studied at a lower level of theory. Those deemed likely to be atmospherically competitive were then calculated using the multi-conformer transition states theory approach developed by Møller et al. (J. Phys. Chem. A, 120, 51, 10072-10087, 2016). This approach has been shown to agree with the experimental values to within a factor of 4 for other systems.

It was found that double bonds are key to fast unimolecular reactions in the first-generation monoterpene hydroxy peroxy radicals. The H-shift reactions abstracting a hydrogen from a carbon adjacent to a double bond are found to typically be fast enough to compete with the bimolecular reactions, likely due to the resonance stability of the nascent allylic radical. The reactivity of the cyclization reaction between the carbon-carbon double bonds and the peroxy group, which forms an endoperoxide ring, is high as well. The H-shifts abstracting the hydrogen from the hydroxy group may be competitive in some cases but the reaction rate coefficients for these reactions are more uncertain. Generally, the cyclization reaction and the allylic H-shift reactions are the dominant reaction paths for the studied peroxyl radicals. Since the OH radical addition consumes one double bond, we suggest that the monoterpenes with more than one double bond in their structure are likely to have unimolecular reactions that can be important for the first-generation monoterpene peroxy radicals. On the other hand, the ones with only one double bond initially are not likely to have fast unimolecular reactions that can compete with the bimolecular reactions under the atmospheric condition, unless a double bond can be formed during their oxidation process as found for α-pinene and β-pinene. This result greatly limits the amount of potentially important unimolecular reaction paths in atmospheric monoterpene oxidation.