Effect of Multistructural Anharmonicity and Tunneling on H Abstraction from Propanal by H, OH, CH3, and C2H5

Abstract

In hydrocarbon fuel oxidation, propanal is a key intermediate and oxygenated pollutant. Its combustion rate constants are mostly estimated by analogy, bringing large uncertainties to kinetic models. This work comprehensively investigates H-abstraction from propanal by H/CH₃/C₂H₅/OH radicals. High-precision rate constants and branching ratios were determined by multistructural variational transition state theory with small-curvature tunneling across 240–2000 K. Our findings emphasize that conformational flexibility and torsional anharmonicity greatly affect both the rate constants and branching ratios. Across the full temperature range, α-site abstraction (R1α) is the primary process in the propanal + H system. For propanal + CH₃, the rate constant of β-site channel (R2β) is dominant. In the propanal + C₂H₅ system, the α-site reaction (R3α) remains prominent, whereas for propanal + OH, the branching ratios of the β-site and γ-site channels (R4β, R4γ) increase with temperature, while that of the α-site (R4α) decreases. Excellent agreement with available experimental data is achieved by the fitted expression k_R4 = 0.0017 × T^4.835 exp(2621.61/T) (in cm³ mol^–1 s^–1) for the total rate constant of propanal + OH. The new rate constants were adopted to update Veloo et al.’s kinetic model, followed by sensitivity analysis. The revised model enables more reliable simulation of propanal combustion and emission behaviors.