Multi-Structural Kinetics Study on H-atom Abstraction from Fuel Molecules by ṄH2 Radicals with Anharmonicity, Recrossing and Tunneling Effects

Abstract

Hydrogen atom abstraction (HAA) reactions by ṄH2 radicals from fuel molecules are critical in ammonia combustion chemistry, particularly in the co-combustion of ammonia with high reactivity fuels, as these C-N cross-reactions play a pivotal role in the development of ammonia blend fuel mechanisms. This study explored the influence of multi-structural effects on the kinetics and thermodynamics of HAA reactions from a range of representative alkanes (n-butane, iso-butane, n-pentane, iso-pentane, n-heptane, and iso-octane) and oxygenated species (butanol, methyl propyl ether and ethyl ethanoate). Rate constants were determined using on-the-fly canonical variational transition-state theory with small-curvature tunneling and multi-structural torsional anharmonicity. The calculated kinetics data were compared with previous reported results obtained from traditional transition state theory with unsymmetrical Eckart tunneling and the 1-D hindered rotor treatment. Additionally, this study investigated the effects of recrossing corrections and small-curvature tunneling, revealing the differences between different tunneling treatment and the influence of recrossing on reaction kinetics. These findings provide critical insights into reaction mechanisms and offer alternative kinetic data for advancing ammonia-hybrid combustion models. The work emphasizes the necessity of accounting for anharmonicity and multi-structural effects in rate constants and thermochemistry calculations, particularly for relative larger molecules with multiple conformers.