Kinetics and thermodynamics of linear C2-C5 aldehyde radicals: from Ab Initio calculations to combustion modeling

This work presents a comprehensive study into the isomerization and decomposition kinetics, and thermodynamic properties of linear C2-C5 aldehyde radicals. Optimized geometries and vibrational frequencies were calculated at the M06-2X/6–311++G(d,p) level of theory, while single-point energies were obtained using the CCSD(T) and MP2 methods combined with basis set extrapolation. Temperature- and pressure-dependent rate constants were systematically calculated in 400–2000 K and at 0.1–100 atm using conventional transition state theory with Eckart tunneling corrections. The influence of carbon chain length on reaction barriers and kinetics was revealed. Among the studied radicals and relevant pathways, acyl-R radicals (C n H 2n+1 CO, n = 1∼4) are the most stable and their α-scission yielding CO and alkyl radicals (C n H 2n+1 ) exhibit the lowest energy barrier and are the dominant decomposition channel, whereas isomerization reactions preferentially proceed via a six-membered-ring transition state. The α-scission reaction of CH 3 ĊO shows relatively high barrier height (16.46 kcal/mol), while those of C3-C5 acyl-R radicals are ca. 14.4 kcal/mol and insensitive to carbon chain length. Similarly, the carbon chain length exhibits a negligible effect on other analogous isomerization and β-scission reactions. Among the studied reactions, the α-scission of acyl-R radical shows the highest rate constants and kinetically dominates in the studied temperature range. The rate rules for isomerization and decomposition reactions from different sites of aldehyde radicals were obtained. Thermochemical properties of the aldehyde radicals were calculated using the single-structural torsional anharmonicity partition function method. Incorporating the newly obtained kinetic and thermodynamic data into detailed kinetic models of C3-C5 aldehydes significantly improves the predictive performance for ignition delay times and speciation evolution of key intermediates during fuel oxidation, such as ethylene and carbon dioxide. This study provides accurate kinetic and thermodynamic data for aldehyde radicals and facilitates the development of more robust kinetic models.