Oligoterpenes Oxidation: Integrating Thermal Analysis Experiments and Ab Initio Kinetics

The influence of isoprene unit number on the reactivity and selectivity of multiphase autoxidation was investigated using evolved gas analysis with coupled TGA/STI/GC-MS experiments on pristane and squalane. Product formation pathways were deciphered using a novel first-principles-augmented kinetic scheme for liquid-phase pristane oxidation, coupled to a detailed gas-phase mechanism through a reaction-evaporation model. Ab initio calculations revealed that the RH-RO2 complex becomes increasingly conformationally intertwined as the backbone elongates, giving rise to a delicate balance between steric, electrostatic, and orbital interactions that govern the Bolland reaction. Two previously unreported ROOH decomposition channels were identified: (i) a low-barrier, bimolecular, cage-like "dissociative abstraction" pathway and (ii) a high-barrier radical Hock rearrangement. Liquid-phase routes to carboxylic acids, including Goosen's pathway and acyl oxidation, were revisited. The resulting phenomenological kinetic model successfully predicts the dominant product peaks qualitatively, indicating a low-temperature route for aromatic formation via branched-alkene recombination. Consistent with the ab initio data, the bimolecular ROOH decomposition pathway is significant only below 130 °C. Acid liquid-phase formation originates mostly from both Korcek and aldehyde+HO2 radical pathways. Above 225 °C, the gas-phase oxidation of the partially oxidized mixture contributes markedly to both product generation and consumption. We uncover bimodal product distributions in squalane, related to the emergence of a specific second oxidation phase. The integrated framework clarifies the oxidation network of isoprenoid oligomers, identifies key reaction channels, and reduces uncertainty in acid formation kinetics. These insights enhance the predictive reliability of large-scale kinetic models, thereby informing strategies for safer fuel/lubricant storage and the rational design of additives to tailor autoxidation pathways for specific applications.