Revealing the mechanism of nicotine pyrolysis: Insights from DFT calculations

Abstract

Nicotine is the most abundant alkaloid in tobacco, so it is of great significance to reveal its pyrolysis mechanism for further understanding of tobacco pyrolysis mechanism. The mechanistic pathways for nicotine decomposition were constructed according to density functional theory (DFT) calculations and pyrolysis experiments. The M062X/6–31 +G (d, p) method and basis set were selected for performing calculations. The results indicated that there were four bonds in nicotine molecule that were susceptible to cleavage with the order of: C7-N11 > C10-N11 > C12-N11 > C4-C7. Combining previous pyrolysis experiments, four possible pathways for nicotine pyrolysis under a radical system were analyzed, and the activation energy of each pathway was compared to determine the most probable reaction. After the cleavage of C7-N11 and C10-N11 bonds, low molecular weight hydrocarbons and some nitrile compounds were generated. C12-N11 bond cleavage produced more complex substances, such as 2-methyl-1H-pyrrole, 7H-cyclopentapyridine, and quinoline, which required the highest activation energy. C4-C7 bond cleavage led to the formation of pyridine, substituted pyridines, and pyrrole. Among them, the binding energy of 3-pyridinecarbonitrile was the highest (−137.75 kcal/mol). Finally, the mechanistic pathways for nicotine decomposition were validated through nornicotine pyrolysis. It revealed that the highest relative content of 3-ethenylpyridine and relatively low content of quinoline in nornicotine pyrolysis products were consistent with the activation energy analysis for nicotine decomposition pathways, which further demonstrated the theoretical and experimental support for the mechanistic pathways of nicotine decomposition.