Structures, energetics, and kinetics of H-atom abstraction from methyl propionate by molecular oxygen: Ab initio and DFT investigations

Initial degradation of methyl propionate (MePr) through its reaction with triplet molecular (³Σ_g⁻) O₂ was investigated using density functional theory (DFT) and ab initio CBS-QB3 calculations over temperature range 700–1600 K. Thermochemistry and kinetics of three hydrogen atom abstractions have been studied. These reactions encounter energy barriers of 44.17–48.10 kcal mol⁻¹ at CBS-QB3. Rate coefficients of the studied channels were evaluated from transition state theory (TST) with tunneling correction. From kinetic and thermodynamic perspectives, the most favorable process is H-atom abstraction from the C_α position, followed by C_μ, and then C_β position. This order is partially different for the reaction of MePr with singlet (¹Δ_g) O₂ where H-atom abstraction from the C_μ atom is the most preferable pathway, followed by C_α position, then C_β position. The atom in molecules (AIM) theory in critical point and natural bond orbital (NBO) analysis was used to explain the existence of covalent interactions.