PMC

The proposed scheme for the formation of the ELVOC monomer (C₁₀H₁₄O₇) and dimer (C₂₀H₃₀O₁₄) surrogates selected for quantum chemical calculations () is based on recently established autoxidation mechanisms for a series of cycloalkane + O₃ systems^,,,,,. Peroxy radicals in the figure are indicated by a green label, E₁ by a red label and E₂ by a blue label. Addition of ozone to the double bond of α-pinene produces two carbonyl-substituted Criegee biradicals. The energy-rich Criegee biradical is either collisionally stabilized, or isomerizes via 1,4-H-shift to a vinylhydroperoxide (VHP), which then decomposes to yield an OH· and an alkenoxy radical. The alkenoxy radical reacts with O₂, leading to a peroxy radical, which is the potential precursor to a sequence of autoxidation reactions leading to the formation of HOMs. Here the peroxy radical C₁₀H₁₅O₄· is chosen as the starting point for HOM formation. The first intramolecular hydrogen abstraction is likely to take place at the aldehydic carbon from the opposite side of the peroxy group, although the rigid four-carbon-atom ring could hinder bending of the structure. For the cis configuration where the peroxy group and the aldehydic hydrogen are on the same side of the cyclobutyl ring, the 1,7-H shift rate is calculated to be 0.14 s⁻¹, which initiates the autoxidation chemistry on a fast timescale compared to the HOM lifetime resulting from loss to the CLOUD chamber walls (about 900 s). The resultant acylic radical undergoes rapid O₂ addition, leading to an -OOH functionalized peroxyacyl radical (C₁₀H₁₅O₆·). The second intramolecular hydrogen abstraction is expected to proceed at the carbon atom in the α position of the peroxyacyl group via 1,4-H isomerization. The resultant C₁₀H₁₅O₈· terminates by known reactions of peroxy radicals (HO₂· or RO₂· under the present experimental conditions), producing a spectrum of HOM monomers that includes the E₁ surrogate, C₁₀H₁₄O₇. The homogeneous recombination of two peroxy radicals via elimination of O₂ produces the covalently bound dimer C₂₀H₃₀O₁₄ chosen as the E₂ surrogate. Alternatively, C₁₀H₁₅O₈· can undergo further autoxidation, if sufficiently labile hydrogen atoms are available, leading to the observed closed-shell monomers with ≥9 O (). The self/cross-reaction of the C₁₀H₁₅O₄· peroxy radical produces an alkoxy radical, which decomposes rapidly, leading to a carbonyl-functionalized peroxy radical (C₁₀H₁₅O₅·). This peroxy radical is another potential starting structure for HOM formation. The carbon-ring-opening reaction pathway, while increasing the steric availability of the H atom, might be a slow step. The effective formation rate of the C=O-functionalized peroxy radical is calculated to be less than about 10⁻³ s⁻¹, which is comparable to its wall deposition rate. The timescale with respect to the subsequent autoxidation reaction, on the other hand, is expected to be of the order of seconds, by analogy with that for branched-chain peroxy radicals. The unbalanced sources and sinks potentially account for the low signals of peroxy radicals with odd oxygen numbers (for example, C₁₀H₁₅O₅, C₁₀H₁₅O₇ and C₁₀H₁₅O₉). The autoxidation process of C₁₀H₁₅O₅· is presumed to proceed by an autoxidative reaction pathway similar to that for C₁₀H₁₅O₄·, eventually leading to the spectrum of HOM monomers and dimers observed in the CLOUD chamber. Except for the autoxidation channel, all the peroxy radicals are still subject to well-established reactions such as R(′)O₂ + RO₂/HO₂, which are potentially important if the reaction rate is comparable to that for the autoxidation channel.

(a) CH₃CH₂OO, (b) CH(O)CH₂OO, and (c) CH₃C(O)OO peroxy radicals.

Transition of two-electron reduced mixed-valence CO complex of CcO to the “peroxy” intermediate.

Total ion chromatogram obtained from peroxy sesquiterpenoids (1 and 2).