Abstract: The gas phase reaction mechanism of Br2+Cl2=2BrCl had been investigated by (U)MP2 at 6-311G* level, and a series of four-centered and three-centered transition states were obtained. Comparing the activation energies of three reaction paths, the result was achieved as follows: the minimum activation energy of the bimolecular elementary reaction Br2+Cl2=2BrCl was less than the dissociation energies of Br2 and Cl2. It was thus theoretically proved that the titled reaction occurred more easily in the bimolecular form without light initiation. However, if the reaction system was initiated by light, the titled reaction was completed quickly through the reaction path of Br atom reacting with Cl2 or Cl atom reacting with Br2. The theoretical result was further verified by determination of the absorbency (with or without light initiation) at 412 nm.

Key words: Bromine, Chlorine, Bromine chloride, Transition state, Reaction mechanism, Spectrophotometry