The Reaction Between the Bromine Atom and the Water Trimer: High Level Theoretical Studies

Three different reaction pathways are found for the reaction of bromine atom (Br) with the lowest-energy structure of the water trimer [uud-(H2O)3], initially using the MPW1K-DFT method. The three bromine pathways have closely related geometries and energetics, analogous to those found for the fluorine and chlorine reactions. The lowest-energy pathway of the Br + uud-(H2O)3 reaction was further investigated using the "gold standard" CCSD(T) method and the correlation-consistent basis sets up to cc-pVQZ(-PP). Based on the CCSD(T)/cc-pVQZ(-PP)//CCSD(T)/cc-pVTZ(-PP) results, the Br + (H2O)3 reaction is endothermic by 33.3 kcal mol-1. The classical barrier height is 29.0 kcal mol-1 between the reactants and the exit complex, and there is no barrier for the reverse reaction. The Br⋯(H2O)3 entrance complex is found to lie 4.7 kcal mol-1 below the separated reactants, and the HBr⋯(H2O)2OH exit complex is bound by 6.4 kcal mol-1 relative to the separated products. This potential energy profile is further corrected by the zero point energies and spin-orbit coupling effects. Structurally, the Br + (H2O)3 stationary points can be derived from those of the simpler Br + (H2O)2 reaction by judiciously appending a H2O molecule. The Br + (H2O)3 potential energy profile is compared with the Br + (H2O)2 and Br + H2O reactions, as well as to the valence isoelectronic Cl + (H2O)3 and F + (H2O)3 systems. It is reasonable to expect that the reactions between the bromine atom and larger water clusters would be similar to the Br + (H2O)3 reaction.