Reaction pathways leading to HPALD intermediates in the OH-initiated oxidation of isoprene

In this study, we revisited the mechanism of isoprene oxidation by OH radicals, focusing on the formation of hydroperoxyaldehydes (HPALDs) in the reactions following O₂-addition at the α-position to Z,Z'-OH-allyl radical products of the 1,6-H shift of the 1st-generation Z-δ-OH-isoprenylperoxy radicals. Utilizing high-level ab initio quantum chemical calculations and a master equation approach, we provide theoretical confirmation that the formation of δ-HPALDs dominates by far and show that production of β-HPALDs by the mechanism proposed by Wennberg et al. (Chem. Rev., 2018, 118, 3337-3390) is negligible. Besides the dominance of the δ-HPALD formation channel, our investigation also reveals a novel though minor reaction channel resulting in the formation of an allylic δ-hydroperoxy acid and OH radical. Of primary importance for the assessment of the respective channels is the identification of a chemically activated mechanism driving the δ-HPALD formation process under atmospheric conditions. Different from traditional thermally activated pathways, we found that the rovibrationally hot peroxy radicals resulting from O₂ addition to Z,Z'-OH-allyl radicals undergo prompt rearrangement and decomposition at a rate faster than their collisional relaxation, predominantly yielding δ-HPALDs in a chemically activated manner with high efficiency under atmospheric conditions.