Reaction mechanism and kinetics of n- and t-butanol with the formyl radical HCO: Site-specific hydrogen abstraction.

CONTEXT: Despite the important role of the formyl radical (HCO) in combustion oxidation networks, its reactions with alcohols remain poorly understood. In this work, the mechanisms and kinetics of HCO reactions with n- and t-C4H9OH were investigated theoretically. For n-butanol, five hydrogen-abstraction pathways corresponding to O-H, α-, β-, γ-, and terminal C-H abstraction were identified, whereas two competing pathways were found for t-C4H9OH. The calculated activation barriers range from 18.5 to 21.8 kcal/mol at the CCSD(T)//DFT level. Among the n-butanol channels, α-H abstraction is strongly favored, with branching ratios of 0.93-0.99 over the temperature range 300-2500 K. For t-butanol, C-H abstraction dominates with a branching ratio of 0.65 at 300 K and becomes even more predominant with increasing temperature, reaching 0.97 at 2500 K. Quantum tunneling significantly enhances the rate constants at low temperatures, while hindered rotor corrections affect the magnitude of the calculated rate constants without changing the dominant reaction pathways. METHODS: Geometries and vibrational frequencies were calculated using the B3LYP and M06-2X methods with the 6-311 + G(d,p) basis set. Electronic energies were refined by CCSD(T) single-point calculations, and rate constants were evaluated using conventional transition state theory including Eckart tunneling and hindered rotor corrections over the temperature range 300-2500 K.