Unveiling the Reaction Mechanism of the N(<sup>2</sup>D) + Pyridine\nReaction: Ring-Contraction versus 7‑Membered-Ring\nFormation Channels

Despite\nthe relevance of the reactions of the prototypical nitrogen-containing\nsix-membered aromatic molecule (N-heterocyclic) of pyridine (C<sub>6</sub>H<sub>5</sub>N) in environmental science, astrochemistry,\nplanetary science, prebiotic chemistry, and materials science, few\nexperimental/theoretical studies exist on the bimolecular reactions\ninvolving pyridine and neutral atomic/molecular radicals. We report\na combined experimental and theoretical study on the elementary reaction\nof pyridine with excited nitrogen atoms, N(<sup>2</sup>D), aimed at\nproviding information about the primary reaction products and their\nbranching fractions (BFs). From previous crossed molecular beam (CMB)\nexperiments with mass-spectrometric detection and present synergistic\ncalculations of the reactive potential energy surface (PES) and product\nBFs we have unveiled the reaction mechanism. It is found that the\nreaction proceeds via N(<sup>2</sup>D) barrierless addition to pyridine\nthat, via bridged intermediates followed by N atom “sliding”\ninto the ring, leads to 7-membered-ring structures. They further evolve,\nmainly via ring-contraction mechanisms toward 5-membered-ring radical\nproducts and, to a smaller extent, via H-displacement mechanisms toward\n7-membered-ring isomeric products and their isomers. Using the theoretical\nstatistical estimates, an improved fit of the experimental data previously\nreported has been obtained, leading to the following results for the\ndominant product channels: C<sub>4</sub>H<sub>4</sub>N (pyrrolyl)\n+ HCN (BF = 0.61 ± 0.20), C<sub>3</sub>H<sub>3</sub>N<sub>2</sub> (1H-imidazolyl/1H-pyrazolyl) + C<sub>2</sub>H<sub>2</sub> (BF =\n0.11 ± 0.06), and C<sub>5</sub>H<sub>4</sub>N<sub>2</sub> (7-membered-ring\nmolecules or pyrrole carbonitriles) + H (BF = 0.28 ± 0.10). The\nring-contraction product channels C<sub>4</sub>H<sub>4</sub>N (pyrrolyl)\n+ HCN, C<sub>3</sub>H<sub>3</sub>N<sub>2</sub> (1H-imidazolyl) + C<sub>2</sub>H<sub>2</sub>, C<sub>3</sub>H<sub>3</sub>N<sub>2</sub> (1H-pyrazolyl)\n+ C<sub>2</sub>H<sub>2</sub>, and C<sub>5</sub>H<sub>5</sub> (cyclopentadienyl)\n+ N<sub>2</sub> have statistical BFs of 0.54, 0.09, 0.11, and 0.07,\nrespectively. Among the H-displacement channels, the cyclic-CHCHCHCHNCN\n+ H channel and cyclic-CHCHCHCHCN<sub>2</sub> + H are theoretically\npredicted to have a comparable BF (0.07 and 0.06, respectively), while\nthe other isomeric 7-membered-ring molecule + H channel has a BF of\n0.03. Pyrrole-carbonitriles and 1H-ethynyl-1H-imidazole (+ H) isomeric\nchannels have an overall BF of 0.03. Implications for the chemistry\nof Saturn’s moon Titan and prebiotic chemistry, as well as\nfor understanding the N-doping of graphene or carbon nanotubes, are\nnoted.