Computational investigation into the gas-phase ozonolysis of the conjugated monoterpene α-phellandrene

Abstract

Reaction with ozone is a major atmospheric sink for α-phellandrene, a monoterpene found in both indoor and outdoor environments, however experimental literature concerning the reaction is scarce. In this study, high-level G4(MP2) quantum chemical calculations are used to theoretically characterise the reaction of ozone with both double bonds in α-phellandrene for the first time. Results show that addition of ozone to the least substituted double bond in the conjugated system is preferred. Following addition, thermal and chemically activated unimolecular reactions, including the so-called hydroperoxide and ester or 'hot' acid channels, and internal cyclisation reactions, are characterised to major first generation products. Conjugation present in α-phellandrene allows two favourable Criegee intermediate reaction pathways to proceed that have not previously been considered in the literature; namely a 1,6-allyl resonance stabilised hydrogen shift and intramolecular dioxirane isomerisation to an epoxide. These channels are expected to play an important role alongside conventional routes in the ozonolysis of a-phellandrene. Computational characterisation of the potential energy surface thus provides insight into this previously unstudied system, and will aid future mechanism development and experimental interpretation involving α-phellandrene and structurally similar species, to which the results are expected to extend.

Ozonolysis of α-phellandrene – Part 2: Compositional analysis of secondary organic aerosol highlights the role of stabilised Criegee intermediates

“…S1). Given vapour pressures are likely to be very similar (Table 2), this observation suggests that either initial attack at the least substituted double bond in a-phellandrene is favoured, or the CI formed from ozonolysis of the more substituted double bond in α-phellandrene is more likely to participate in the hydroperoxide channel than the CI from ozonolysis of the less substituted double bond, both of which are consistent with recent theoretical findings (Mackenzie-Rae et al, 2016). 20…”

“…Both of these are major reaction pathways and can explain the high abundance of 25 compound N8 in α-phellandrene SOA. Theoretical results show the energy barrier for accessing the hydroperoxide channel from the primary CI generated from the more substituted double bond is around 20 kJ mol -1 lower in energy than the respective barrier for accessing the hydroperoxide channel from the relevant CI from the less substituted double bond (Mackenzie-Rae et al, 2016). Based on this information, the secondary-alcohol isomer (formation mechanism shown in Fig. 2) is favoured as compound N8.…”

“…1 with major peaks detected across all experiments labelled in 10 retention time order. Using high mass accuracy tandem mass spectra, the structure of 12 major compounds have been identified (Table 2), with structural elucidation consistent with postulated degradation mechanisms (Mackenzie-Rae et al, 2016, 2017a. Labelled tandem mass spectra are provided in the Supplement (Sect.…”

“…Because all first-and second-generation products of α-phellandrene ozonolysis contain at least one functional group that is capable of ionisation (Mackenzie-Rae et al, 2016, 2017a, it is reasonable to assume that a high proportion of water soluble SOA components will be observed, with the analyte R being observed as [R-H] -ion in the negative mode and [R+Na] + ion 25 in the positive mode. Negative mode analysis leads to formation of deprotonated ions; hence molecules containing functional groups that readily lose a proton, such as carboxylic acids, are frequently observed in this mode.…”

“…4. The mechanism however, which was first proposed by Ma et al (2007) to explain pinonic acid formation from a-pinene, was shown to be uncompetitive with competing pathways by computational calculations (Mackenzie-Rae et al, 2016). An alternative prospect is that aldehyde groups are oxidised to carboxylic acids, increasing the mass by 16 Da, in a process that has been proposed to occur non-negligibly in the aerosol phase (Walser et al, 30 2008).…”