Understanding key interactions between NOx and C2-C5 alkanes and alkenes: The ab initio kinetics and influences of H-atom abstractions by NO2

Abstract

This study aims to reveal the important role and the respective rate rules of H-atom abstractions by NO₂ for better understanding NO_X/hydrocarbon interactions. To this end, H-atom abstractions from C₂-C₅ alkanes and alkenes (15 species) by NO₂, leading to the formation of three HNO₂ isomers (trans-HONO, HNO₂, and cis-HONO) and their respective products (45 reactions), are first characterized through quantum chemistry computation, where electronic structures, single point energies, C-H bond dissociation energies and 1-D hindered rotor potentials are determined at DLPNO-CCSD(T)/cc-pVDZ//M06–2X/6−311++g(d,p). The rate coefficients for all studied reactions, over a temperature range from 298.15 to 2000 K, are computed using transition state theory with the Master Equation System Solver program. Comprehensive analysis of branching ratios elucidates the diversity and similarities between different species, HNO₂ isomers, and abstraction sites, from which accurate rate rules are determined. With the rate rules, the rate coefficients at various reaction sites on heavier hydrocarbons (e.g., > C₅) can be reliably estimated by analogy. Incorporating the updated rate parameters into a detailed chemical kinetic model reveals the significant influences of this type of reaction on model prediction results, where the simulated ignition delay times are either prolonged or reduced, depending on the original rate parameters presented in the selected model. Sensitivity and flux analysis further highlight the critical role of this type of reaction in affecting system reactivity and reaction pathways, emphasizing the need for adequately representing these kinetics in existing chemistry models. This can now be sufficiently achieved for alkanes and alkenes based on the results from this study.