Unraveling Addition, Cyclization, and Dehydrogenation Reactions between Pyridinyl Radicals and Acrylonitrile Using Synchrotron VUV Photoionization Mass Spectrometry and Theoretical Insights.

In this study, we tackle the challenge of synthesizing nitrogen-containing bicyclic compounds in the gas phase, a topic of considerable interest to interstellar chemistry and combustion science. We specifically explore the bimolecular reaction between the o-pyridinyl radical (C5H4N•), a quintessential nitrogen-containing monocyclic carbon-centered aromatic radical, and the interstellar molecule acrylonitrile (C3H3N) in a resistance-heated silicon carbide (SiC) microreactor. Utilizing advanced molecular beam-sampled synchrotron VUV photoionization mass spectrometry (SVUV-PIMS), we detected the formation of C8H6N2 species at 1100 K. Photoionization efficiency (PIE) curve analysis suggested the exclusive production of a ring-tail structure, rather than the desired bicyclic 1,5-naphthyridine structure. High-level ab initio calculations revealed a detailed reaction mechanism that addition, cyclization, and subsequent dehydrogenation reactions between C5H4N• and C3H3N can lead to the formation of either 3-(2-pyridinyl)-2-propenenitrile or 1,5-naphthyridine. However, the formation of 1,5-naphthyridine under the present experimental conditions was not efficient, as supported by rate coefficient calculations. The complex mechanisms of radical-neutral reactions were revealed in this work, guiding the gas-phase synthesis of nitrogen-containing aromatic compounds.