Theoretical Study on Reaction Mechanism of Ground-State Cyano Radical with 1,3-Butadiene: Prospect of Pyridine Formation

The reaction of ground-state cyano radicals, CN­(X²Σ⁺), with the simplest polyene, 1,3-butadiene (C₄H₆(X¹A_g)), is investigated to explore probable routes and feasibility to form pyridine at ultralow temperatures. The isomerization and dissociation channels for each of the seven initial collision complexes are characterized by utilizing the unrestricted B3LYP/cc-pVTZ and the CCSD­(T)/cc-pVTZ calculations. With facilitation of RRKM rate constants, through ab initio paths composed of 7 collision complexes, 331 intermediates, 62 hydrogen atom, 71 hydrogen molecule, and 3 hydrogen cyanide dissociated products, the most probable paths at collision energies up to 10 kcal/mol, and thus the reaction mechanism, are determined. Subsequently, the corresponding rate equations are solved that the concentration evolutions of collision complexes, intermediates, and products versus time are obtained. As a result, the final products and yields are determined. The low-energy routes for the formation of most thermodynamically stable product, pyridine, are identified. This study, however, predicts that seven collision complexes would produce predominately 1-cyano-1,3-butadiene, CH₂CH­CHCH­CN (<b>p2</b>) plus atomic hydrogen via the collision complex <b>c1</b>(CH₂CH­CHC­H₂CN) and intermediate <b>i2</b>(CH₂CH­CH₂CH­CN), with a very minor amount of pyridine. Our scheme also effectively excludes the presence of 2-cyano-1,3-butadiene, which has energy near-degenerate to 1-cyano-1,3-butadiene, as supported by experimental findings