Ab initio kinetics of OH-initiated oxidation of pyridine: new insights into nitrogen-included aromatic rings

Pyridine, a common component of coal and a well-established model for studying the mechanisms and kinetics of nitrogen compounds, has been extensively investigated for four decades. The reaction of pyridine with OH is also a focal point of research due to the importance of OH in both atmospheric and combustion conditions. However, previous studies on the reaction of pyridine with OH have yet to be consistent in elucidating the mechanism and have provided insufficient kinetic models to thoroughly predict the possible transformation of pyridine and to unravel the effect of nitrogen on the reactivity of the aromatic ring under different conditions. By constructing a more comprehensive kinetic mechanism model for the title reaction, we have determined that the formation of ortho-C₅H₄N (P1) + H₂O is dominant and pressure-independent at both low and high temperatures. This finding is validated through a comparison of barrier heights, bond dissociation energy, and kinetic analysis. We also demonstrate that the introduction of nitrogen into the aromatic ring (pyridine) results in a pressure-independent trend, with the lowest reactivity observed at low temperatures and the highest reactivity at high temperatures when compared to a pure aromatic ring (benzene) and the system having N adjacent to the aromatic ring (aniline). Furthermore, it is found that H-abstraction consistently dominates in nitrogen-containing aromatics (pyridine, diazine isomers, and 1,3,5-triazine), resulting in lower reactivity compared to pure benzene ring at low temperatures and higher reactivity at high temperatures (except for pyrazine), and exhibiting no pressure dependence. Such insights are considered valuable for understanding the role of nitrogen compounds in energetic materials and controlling the formation of toxic byproducts such as HCN or NO during combustion.