Ether and ester formation from peroxy radical recombination

In a recent combined experimental and computational investigation (Peräkylä et al, JACS 145,35 7780–7790, 2023) of the Peroxy Radical Recombination (RO2 + RO2) products of α-pinene, we discovered a previously unknown product channel, in which the expected attachment of the two alkoxy radical (RO) intermediates into a peroxide accretion product (ROOR) is preceded by a rapid decomposition of one of the intermediate RO into an acyl-centered radical, resulting in the formation of a smaller but more stable ester accretion product R’(O)COR. In the presented work, the atmospheric implications of this new reaction channel have been explored further using a modified version of the GECKO-A (Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere, Aumont et al, ACP 5, 2497–2517, 2005) to generate a large representative sample of RO2 + RO2 reactive pairs from the atmospheric oxidation of n-Decane, Toluene, the seven most common Monoterpene molecules) and one Sesquiterpene along with all the known decomposition channels of the intermediate RO formed in the reactions. The reaction rates of these decomposition channels are then compared to previous theoretical work on the product branching from RO2 + RO2 reactions (Hasan et al. JPCA, 124, 8305–8320, 2020 & 127, 1686–1696, 2023; Franzon JPCA 127, 5956–5966, 2023) to determine for which systems these RO decompositions might be competitive.

The generated chemical data is discussed in terms of the atmospheric formation of low-volatility organic molecules. Data is presented on the most important RO decomposition reactions for ether and ester accretion product formation, on vapour pressure trends of the various products, and on new accretion product-inhibiting reaction channels that at best produce two closed-shell molecules with the same carbon count as the reactant RO2.

Since our work is exploratory, and as the calculation of reaction rates rests on many assumptions, no certain conclusions can be drawn from our calculated product branching ratios. However, the present work provides valuable new insights on the formation of low-volatility organics in the atmosphere, and raises many open questions worthy of detailed studies of their own. We hope our results will be of great general interest to the atmospheric chemistry and physics community.