The theoretical study on gas phase photochemical chlorination mechanism of pyridine

The gas phase additional mechanism of the photochemical reaction of pyridine leading to 2-chloropyridine, 3-chloropyridine and 4-chloropyridine has been theoretically studied by DFT B3LYP/3-21G*. The results show that the energy of transition state for producing 2-chloropyridine is the lowest among three reaction pathways, and also the activation energy of resulting 2-chloropyridine is the lowest one, which is -704.830027 a.u. and 114.60 kJ/mol, respectively. So 2-chloropyridine should be a major product, which is in agreement with experimental results. The transition state, activation barriers, IRC analysis and thermodynamic and kinetic studies for ortho-path are performed at B3LYP/6-31G* level. The result shows that. during the process of the reaction, the breaking of C-H and the formation of C-Cl are concerted but not synchronous, and with the constant adjustment of the system structure, the net atomic charge of reactive position varies obviously. The thermodynamic and kinetic studies of the reaction reveal that the reaction take place easily at higher temperatures. (C) 2002 Elsevier Science B.V. All rights reserved.