Anchoring the potential energy surface for the Br + H2O → HBr + OH reaction

The forward and reverse reactions Br + H 2 O → HBr + OH are important in atmospheric and environmental chemistry. Five stationary points on the potential energy surface for the Br + H 2 O → HBr + OH reaction, including the entrance complex, transition state, and exit complex, have been studied using the CCSD(T) method with correlation-consistent basis sets up to cc-pV5Z-PP. Contrary to the valence isoelectronic F + H 2 O system, the Br + H 2 O reaction is endothermic (by 31.8 kcal/mol after zero-point vibrational, relativistic, and spin–orbit corrections), consistent with the experimental reaction enthalpy. The CCSD(T)/cc-pV5Z-PP method predicts that the reverse reaction HBr + HO → Br + H 2 O has a complex but no classical barrier. When zero-point vibrational energies are added, the transition state lies 0.25 kcal/mol above the separated products. This is consistent with the negative temperature dependence for the rate constant observed in experiments. The entrance complex is predicted to lie 2.6 kcal/mol below separated Br + H 2 O. The exit complex is predicted to lie 1.8 kcal/mol below separated HBr + OH.