Density functional theory modeling and time-of-flight secondary ion mass spectrometric and X-ray photoelectron spectroscopic investigations into mechanistic key events of coronene oxidation : toward molecular understanding of soot combustion

Density functional [revised Perdew–Burke–Ernzerhof (rPBE)/double-numerical basis set with polarization functions (DNP)] molecular modeling of the mechanistic steps of coronene oxidation was performed together with X-ray photoelectron spectroscopic (XPS) and time-of-flight secondary ion mass spectrometric (ToF-SIMS) identification of the reaction intermediates. For the reaction steps involving singlet–triplet crossing, transition states were located within the broken-symmetry approach by use of a Becke three-parameter Lee–Yang–Parr (B3LYP)/triple-ζ plus polarization (TZVP) calculation scheme. For the conceivable attack by $O_{2}$ molecule, topologically different in-plane, free edge, zigzag, bay, armchair, and C–H sites of the coronene molecule and its derivatives were considered. Energetic profiles of the oxidation cascade revealed that the reaction occurs by a sequence of oxidation and fragmentation steps. For the key steps, the influence of the evolution of free energy with temperature was assessed and discussed. The most demanding stage is the primary dioxygen attack, and the preferred locus is the coronene free-edge site. The key intermediates appearing during the reaction progress feature C═O, C–OH, and COOH moieties, redistribution of the carbon rings, and formation of new carbon–oxygen rings, as confirmed additionally by corroborative spectroscopic measurements. The obtained results are discussed in the broader context of the molecular mechanism of soot oxidation.