Despite
the relevance of the reactions of the prototypical nitrogen-containing
six-membered aromatic molecule (N-heterocyclic) of pyridine (C6H5N) in environmental science, astrochemistry,
planetary science, prebiotic chemistry, and materials science, few
experimental/theoretical studies exist on the bimolecular reactions
involving pyridine and neutral atomic/molecular radicals. We report
a combined experimental and theoretical study on the elementary reaction
of pyridine with excited nitrogen atoms, N(2D), aimed at
providing information about the primary reaction products and their
branching fractions (BFs). From previous crossed molecular beam (CMB)
experiments with mass-spectrometric detection and present synergistic
calculations of the reactive potential energy surface (PES) and product
BFs we have unveiled the reaction mechanism. It is found that the
reaction proceeds via N(2D) barrierless addition to pyridine
that, via bridged intermediates followed by N atom “sliding”
into the ring, leads to 7-membered-ring structures. They further evolve,
mainly via ring-contraction mechanisms toward 5-membered-ring radical
products and, to a smaller extent, via H-displacement mechanisms toward
7-membered-ring isomeric products and their isomers. Using the theoretical
statistical estimates, an improved fit of the experimental data previously
reported has been obtained, leading to the following results for the
dominant product channels: C4H4N (pyrrolyl)
+ HCN (BF = 0.61 ± 0.20), C3H3N2 (1H-imidazolyl/1H-pyrazolyl) + C2H2 (BF =
0.11 ± 0.06), and C5H4N2 (7-membered-ring
molecules or pyrrole carbonitriles) + H (BF = 0.28 ± 0.10). The
ring-contraction product channels C4H4N (pyrrolyl)
+ HCN, C3H3N2 (1H-imidazolyl) + C2H2, C3H3N2 (1H-pyrazolyl)
+ C2H2, and C5H5 (cyclopentadienyl)
+ N2 have statistical BFs of 0.54, 0.09, 0.11, and 0.07,
respectively. Among the H-displacement channels, the cyclic-CHCHCHCHNCN
+ H channel and cyclic-CHCHCHCHCN2 + H are theoretically
predicted to have a comparable BF (0.07 and 0.06, respectively), while
the other isomeric 7-membered-ring molecule + H channel has a BF of
0.03. Pyrrole-carbonitriles and 1H-ethynyl-1H-imidazole (+ H) isomeric
channels have an overall BF of 0.03. Implications for the chemistry
of Saturn’s moon Titan and prebiotic chemistry, as well as
for understanding the N-doping of graphene or carbon nanotubes, are
noted.
