Ab initio kinetics for H-atom abstraction from C₁–C₅ hydrocarbon and oxygenated species by CH₃Ȯ radicals

Abstract

Hydrogen atom abstraction by methoxy (CH3Ȯ) radicals plays an important role in gasoline/ethanol combustion chemistry. Detailed kinetic reactions for H-atom abstraction by CH3Ȯ radicals from short carbon chain species, including alkanes, alkenes (including vinylic, allylic and diene), alkynes, ethers, ketones, and aldehydes is systematically studied in this work. The M06–2X/6–311++g(d,p) level of theory is used for geometry optimizations, vibrational frequencies calculations, and the hindered rotor treatments for low-frequency modes. QCISD(T)/cc-pVXZ (where X = D and T) and Møller–Plesset perturbation theory MP2/cc-pVXZ (where X = D, T and Q) are used to calculate single point energies. The C–H bond dissociation energies and reaction barrier heights are further analyzed. High–pressure limiting rate coefficients for all hydrogen atom abstraction channels are performed using conventional transition state theory with unsymmetric tunneling corrections. The updated rate coefficients are incorporated into the latest gasoline chemistry model to investigate the influence of these reactions on model performance. The results suggest that the updated model predictions of ignition delay times for acetaldehyde (CH3CHO) and acetone (CH3COCH3) in “air” are considerably affected due to the rich production of CH3Ȯ radicals. This highlights the significance of CH3Ȯ radicals during fuel combustion, particularly at temperatures below 1000 K, which has been overlooked in the past.