Molecular mechanism for rapid autoxidation in α-pinene ozonolysis

Why?

α-pinene is the most emitted monoterpene and a very efficient precursor of secondary organic aerosol (SOA), with reaction (α-pinene + ozone (O₃)) being one of the main SOA-forming systems in the atmosphere. However, the known pathways of the reaction could not fully explain the rapid SOA formation. 

Recently, some VOC have been shown to rapidly undergo autoxidation, a series of sequential peroxy radical (RO₂) hydrogen shifts (H-shifts) and O₂ additions to the hydrocarbon chain, leading to highly oxygenated organic molecules (HOM). In the previously proposed mechanisms, the steric strain of the cyclobutyl ring hindered the H-shifts in the RO₂ isomers. ‍

How?

Quantum chemical calculations of the route that the reaction (α-pinene +O₃) can take followed by a targeted laboratory experiment to confirm the sub-second timescales of the reaction. Laboratory experiments were performed with a Multi-scheme chemical IONization inlet (MION) coupled to the time-of-flight mass spectrometer and using nitrate as a reagent ion source.

Highlighted results:

A new pathway for the α-pinene ozonolysis reaction is proposed that shows a possibility of forming RO₂ without steric strain using the excess energy from the initial reaction. The proposed autoxidation steps include (Fig. 1): a formation of a six-member endoperoxide from the RO₂ via the transition state TS-endo, which subsequently adds an O₂ to form a six-oxygen containing peroxy radical O₆-RO₂. This molecule can undergo a fast 1,4 aldehydic H-shift to form O₆-hydroperoxide via the transition state TS-1,4 H-shift, which subsequently adds an O₂ to form an eight-oxygen containing peroxy radical O8-RO₂.

The speed of the peroxy radical C₁₀H₁₅O₈ formation was explored in the laboratory settings for precursor residence times between 75 and 1000 ms with the inlet flow rates 20 or 30 l per minute. The experiments confirmed the extremely rapid formation of C₁₀H₁₅O₈ – within 75-ms residence time as shown in the Figure 2. The next ten-oxygen containing RO₂ was formed within 300 ms. 

‍Reference: Iyer, S., Rissanen, M.P., Valiev, R. et al. Molecular mechanism for rapid autoxidation in α-pinene ozonolysis. Nat Commun 12, 878 (2021). https://doi.org/10.1038/s41467-021-21172-w 

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