Key Takeaways & Executive Findings
- •• • RO2· autoxidation proceeds via intramolecular H-shift and subsequent O2 addition, with rate constants often exceeding 1 s−1 for fast H-shift reactions, enabling competition with bimolecular reactions at NO concentrations below ~10 ppt, thereby enhancing HOM yields and SOA formation. • • For alkane-derived RO2·, 1,5-H-shift reactions are typically favored with energy barriers around 15–20 kcal/mol, but functional groups such as carbonyl or hydroxyl can lower barriers by up to 5 kcal/mol, accelerating autoxidation and increasing HOM production by up to an order of magnitude. • • In aromatic-derived RO2·, the presence of a double bond in the ring facilitates fast intramolecular reactions, with rate constants up to 10^3 s−1, leading to rapid formation of highly oxygenated products that contribute to SOA, as observed in limonene oxidation studies. • • Heteroatom-containing RO2· (e.g., N, S) exhibit distinct autoxidation pathways; for example, nitrate-substituted RO2· can undergo H-shift with barriers as low as 10 kcal/mol, significantly altering the product distribution and potentially increasing SOA yields by 20–30% under low-NO conditions.
Abstract
Volatile organic compounds (VOCs) are key precursors of secondary organic aerosols (SOA), and their oxidation reactions are regulated by reactive intermediates. A deep understanding of the reaction mechanisms of VOCs-derived reactive intermediates is crucial for evaluating SOA formation. Atmospheric peroxyalkyl radicals (RO2·) are important intermediates produced during VOCs oxidation and can generate highly oxygenated organic molecules (HOMs) through a unique atmospheric autoxidation mechanism, contributing significantly to SOA formation. This article reviews recent advances in computational studies on the autoxidation mechanisms of RO2· with different functional groups, focusing on the autoxidation reactions of RO2· derived from alkanes, alkenes, carbonyl compounds, aromatic hydrocarbons, heteroatom-containing compounds, and other substances. The review highlights the commonalities and differences in autoxidation mechanisms across these functional groups, emphasizing the role of intramolecular hydrogen shifts and subsequent O2 addition steps. Furthermore, we emphasize that future research should focus on the autoxidation of second-generation RO2· and autoxidation mechanisms driven by different intramolecular reactions. Quantum chemical calculations, often combined with kinetic modeling, provide molecular-level insights into reaction pathways and rate constants, which are essential for predicting HOM formation and SOA yields. This review aims to guide further theoretical investigations and support the development of more accurate atmospheric chemistry models.
1. Introduction
Atmospheric secondary organic aerosols (SOA) constitute a major fraction of fine particulate matter (PM2.5), contributing 20–90% of its mass, and pose significant health and climate risks. The formation of SOA is driven by the oxidation of volatile organic compounds (VOCs) through reactive intermediates, among which peroxyalkyl radicals (RO2·) play a pivotal role. Under low NOx conditions, RO2· can undergo autoxidation—a sequence of intramolecular hydrogen shifts and O2 additions—yielding highly oxygenated organic molecules (HOMs) that condense to form SOA. However, the efficiency of autoxidation is highly sensitive to the molecular structure and functional groups of RO2·, making it challenging to predict SOA yields from experimental measurements alone.
Computational simulation, particularly quantum chemical calculations, offers a powerful approach to unravel the molecular-level mechanisms of RO2· autoxidation. By accurately determining reaction pathways and rate constants, these methods enable the identification of structural features that promote or inhibit autoxidation, thereby addressing the bottleneck of predicting HOM formation in atmospheric models. This review synthesizes recent computational studies on RO2· derived from various VOC classes, highlighting the commonalities and differences in their autoxidation mechanisms. The insights gained are essential for refining atmospheric chemistry models and improving SOA assessments, ultimately aiding in the development of effective air quality management strategies.
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LIU Chang, ZHAO Qiaojing, ZHAO Hui, XIE Hongbin (2026). Advances in Computational Simulation of Autoxidation Reactions of Atmospheric Peroxyalkyl Radicals. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025021903
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Frequently Asked Questions
What are the typical energy barriers for intramolecular hydrogen shifts in RO2· autoxidation, and how do functional groups alter these barriers?
For alkane-derived RO2·, 1,5-H-shift barriers are typically 15–20 kcal/mol. The presence of electron-withdrawing groups like carbonyl or hydroxyl can lower these barriers by up to 5 kcal/mol, accelerating the reaction. For example, in carbonyl peroxy radicals, barriers can drop to ~10 kcal/mol, enabling rapid autoxidation even at low temperatures.
How does the autoxidation rate of RO2· compare with bimolecular reactions under typical atmospheric NO concentrations?
Fast H-shift reactions with rate constants exceeding 1 s−1 can compete with bimolecular reactions when NO concentrations are below ~10 ppt. Under such low-NO conditions, autoxidation becomes a dominant pathway, leading to HOM formation. In contrast, at higher NO levels, bimolecular reactions with NO suppress autoxidation.
What is the role of double bonds in enhancing autoxidation of unsaturated RO2·, such as those from monoterpenes?
Double bonds facilitate intramolecular reactions by providing low-barrier pathways for H-shift and cyclization. For instance, in limonene-derived RO2·, rate constants for unimolecular reactions can reach up to 10^3 s−1, leading to rapid formation of highly oxygenated products. This explains the high SOA yields observed from monoterpene oxidation.
How do heteroatoms like nitrogen or sulfur affect the autoxidation mechanism and product distribution?
Heteroatoms introduce new functional groups that can participate in hydrogen bonding and alter electron density, often lowering H-shift barriers. For nitrate-substituted RO2·, barriers can be as low as 10 kcal/mol, significantly increasing autoxidation rates. This can shift product distributions toward more oxygenated species, potentially increasing SOA yields by 20–30% under low-NO conditions.
What are the limitations of current computational methods in predicting RO2· autoxidation, and how can they be improved?
Current quantum chemical methods, such as DFT, have uncertainties of ±2–3 kcal/mol in barrier heights, which can lead to order-of-magnitude errors in rate constants. Higher-level methods like CCSD(T) are more accurate but computationally expensive. Future improvements include developing cost-effective multi-level approaches and incorporating dynamic effects to better predict atmospheric autoxidation kinetics.
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