Ab Initio Kinetics for Hydrogen Abstraction from Aldehydes and Alcohols by CH₃Ȯ Radicals

The process of hydrogen abstraction by methoxy radicals (CH₃Ȯ) represents a fundamental reaction class in hydrocarbon combustion chemistry, playing a pivotal role in fuel decomposition kinetics and radical chain propagation mechanisms. The reaction rate constants for hydrogen abstraction from C₁-C₂ aldehydes and C₁-C₃ alcohols by CH₃Ȯ radicals are systematically studied by using high-level quantum chemical calculations. Geometry optimization, determination of vibrational frequency, and dihedral angle scans are conducted with the M06-2X/6-311++G(d,p) approach. The QCISD(T)/cc-pVXZ (X = D, T) and MP2/cc-pVXZ (X = D, T, and Q) levels of theory are employed for calculating the single-point energies. Rate constants are derived using transition-state theory, which incorporates quantum mechanical effects, while the thermochemical properties are obtained through statistical thermodynamics. Rate comparisons are conducted for abstracting hydrogen from different sites for a given molecule and from a specific site in different molecules. All computational results are subsequently integrated into the NUIGMech1.3 model to evaluate their impact on the prediction of ignition delay times (IDTs). The results indicate that the newly introduced thermodynamic and kinetic parameters have a significant effect on the IDTs of NC₃H₇OH and IC₃H₇OH. Sensitivity and flux analyses are conducted to determine the essential reactions that govern the observed phenomena.