Abstract: The mechanism of reaction Br2＋2HI=2HBr＋I2 has been carefully investigated with density functional theory(DFT) at B3LYP/3-21G** level, and a series of four-centred and three-centred transition states have been obtained. The activation energies of the bimolecular elementary reactions Br2＋HI→HBr＋IBr and IBr＋HI→I2＋HBr(81.02 and 121.08 kJ•mol－1, respectively) are less than the dissociation energy of Br2, HI and IBr(249.21, 320.16, and 232.42 kJ•mol－1). It is thus theoretically proved that the title reaction occurs more easily in the bimolecular form with two medium steps. And it was also found that the reaction of I atom and Br2 to form stable IBr2 molecule is a process without energy barrier, and the energy barrier for decomposition of IBr2 into IBr and Br atom is 70.88 kJ•mol－1.

Key words: Bromine, Hydrogen iodide, Density functional theory(DFT) method, 　Reaction mechanism, Bimolicular reaction