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.