Combined Experimental and Computational Investigation of the Elementary Reaction of Ground State Atomic Carbon (C; 3P j) with Pyridine (C5H5N; X1A1) via Ring Expansion and Ring Degradation Pathways.

We explored the elementary reaction of atomic carbon (C; 3P j) with pyridine (C5H5N; X1A1) at a collision energy of 34 ± 4 kJ mol-1 utilizing the crossed molecular beams technique. Forward-convolution fitting of the data was combined with high-level electronic structure calculations and statistical (RRKM) calculations on the triplet C6H5N potential energy surface (PES). These investigations reveal that the reaction dynamics are indirect and dominated by large range reactive impact parameters leading via barrier-less addition to the nitrogen atom and to two chemically nonequivalent "aromatic" carbon-carbon bonds forming three distinct collision complexes. At least two reaction pathways through atomic hydrogen loss were identified on the triplet surface. These channels involve multiple isomerization steps of the initial collision complexes via ring-opening and ring expansion forming an acyclic 1-ethynyl-3-isocyanoallyl radical (P1; 2A″) and a hitherto unreported seven-membered 1-aza-2-dehydrocyclohepta-2,4,6-trien-4-yl radical isomer (P3; 2A), respectively. For RRKM calculations at zero collision energy, representing conditions in cold molecular clouds, the ring expansion product P3 is formed nearly exclusively for the atomic hydrogen loss channel, but based on these computations, the molecular fragmentation channel forming acetylene (C2H2) plus 3-cyano-2-propen-1-ylidene (P6; 3A″) accounts for nearly all of the degradation products of the reaction of atomic carbon with pyridine, proposing a destruction pathway of interstellar pyridine, which may account for the absence in the detection of pyridine in the interstellar medium. These results are also discussed in light of the isoelectronic carbon-benzene (C6H6; X1A1) system with important implications to the rapid degradation of nitrogen-bearing polycyclic aromatic hydrocarbons (NPAHs) in the interstellar medium compared to mass growth processes of PAH counterparts through ring expansion.