NO and NO2 reactions with oxygenated peroxy radicals lead to indistinguishable product compositions: computational insights from cyclohexene oxidation in presence of NO x.

This study presents a computational investigation into the role of nitrogen oxides (NO x ) in modulating gas-phase cyclohexene oxidation by ozone (O3) and hydroxyl radical (OH) through their reactions with peroxy radicals (RO2) formed during the oxidation sequence. While NO2 suppresses further oxidation by forming peroxynitrates (RO2-NO2), NO can either enhance autoxidation through the formation of reactive alkoxy radicals (RO) or terminate the radical chain via organic nitrate (RO-NO2) formation. In this work, we evaluate the binding affinities of NO x toward RO2 molecules formed at different oxidation stages at DLPNO-CCSD(T)/aug-cc-pVTZ//ωB97X-D/aug-cc-pVTZ level of theory. This is followed by dissociation rate coefficient calculations using detailed balance and master equation simulations, indicating the lifetime and fragmentation pathways of the covalently bound RO2-NO x molecules. The results suggest a tendency toward stronger RO2-NO x stabilization with increasing oxidation stage, with a strong influence from molecular structure. This study also explores the rarely studied dissociation of RO2-NO2 into RO and NO3. Furthermore, by computing the binding enthalpies of NO3 - and with RO2-NO x , this work assesses the detectability of these N-containing oxidation products by the nitrate ion based chemical ionization mass spectrometer (NO3 --CIMS). The results are benchmarked against nitrophenol compounds known to form stable nitrate adducts. This analysis highlights potential detection biases associated with measuring nitrate-functionalized compounds using NO3 --CIMS as variations in molecular structure and clustering tendency influence signal intensities. Overall, these results highlight the regulatory roles of NO and NO2 in hydrocarbon oxidation and offer insights to improve interpretation of these oxygenated organic nitrates in atmospheric CIMS measurements.