The mechanism od photochemical chlorination of pyridine has been investigated using density functional theory (DFT) at the B3LYP/3--- 21G* level. The transition states for three possible reaction paths are obtained and verified by vibration analysis. IRC calculation is carried out for formation of 2-chloropyridine. By comparing the calculated barriers, it is found that the acitvation energy for formation of 2- chloropyridine is 114.60kJ/MOL, which is the lowest among the three reaction pathways. Therefore, the main product should be 2-chloropyridine. This is in agreement with the experimental result. The IRC calculation shows that the breaking of C(2)-----H(7) and the forming of C(2)---Cl(8) are concerted but not synchronous.

Key words: PYRIDINE, TRANSITION STATE THEORY, AB INITIO CALCULATION, REACTION MECHANISM