Study on the thermal oxidation mechanism of oleic acid based on density functional theory.

contextLipid oxidation degrades food quality by producing harmful aldehydes, ketones, and acids. This study integrates density functional theory (DFT) and kinetic analysis to elucidate the thermal oxidation mechanism of oleic acid using a 4-octene model. Mechanistic insights reveal distinct early- and mid-stage processes: the initial phase is dominated by low-barrier allyl-oxygen bonding (< 10.0 kcal/mol), while allyl isomerization (> 60.0 kcal/mol) is negligible. Deep oxidation involves four pathways: 1) O-O• radical attack on adjacent carbons (29.2/11.3 kcal/mol), forming aldehydes; 2) peroxy radical isomerization (rate-limiting carbon jump > 30.0 kcal/mol); 3) epoxide formation via O-O• attack on double bonds (6.8 kcal/mol); 4) hydroperoxide conversion hindered by high barriers (57.3-58.7 kcal/mol). Alkoxy radicals induce C-C bond cleavage, forming minor cyclic epoxides. Kinetic analysis (60 and 150 °C) identifies tricyclic epoxide as the dominant product, stabilized at elevated temperatures.

methodsThis article uses density functional theory (DFT) and Gaussian 16 program to complete structural optimization, frequency analysis, and single point energy calculation. B3LYP-D3/6-31G (d, p) method is used for optimization and frequency calculation, and single point energy correction is performed using the def2TZVPP basis set. The transition state has been verified by IRC, and the free energy has been corrected by Shermo program zero-point energy. The reaction rate constant is based on the transition state theory (TST) and takes into account the tunneling effect, and is calculated using the TST calculator program. Finally, the differential equation system is solved using MATLAB to obtain the time concentration curve of the oxidation product.