An ab initio and experimental study of bromine on low-temperature water clusters and ice surfaces

Abstract

A dual RAIR (reflection–absorption infrared) spectroscopy and ab initio study of the interaction between molecular bromine and water-ice has been performed. The spectra were measured in the temperature range 85 to 180 K and the results compared to theoretical models of Br–H₂O clusters and hydrated H₂OX⁺ and X₂OH⁻ ions. Particular theoretical attention was paid to the possibility that the interacting molecules modify their properties (geometry, partial atomic charge and vibrational frequencies) with the size of the water clusters i.e. Br₂···(H₂O)_n. Additionally, ab initio methods were employed to investigate the actual mechanism of the bromine hydrolysis by evaluating the stability of possible reaction intermediates such as [H₂OBr]⁺ in the gas-phase as well as upon solvation. The chemical structures and vibrational frequencies of the hydrated ion-pairs were also examined. The theoretical treatments employed show that the bromine and water molecules can form a weakly bonded binary complex with interaction between the Br- and O-atoms. Furthermore, the experimental evidence obtained for bromine hydrolysis occurring via an [H₂OBr]⁺ ion on water-ice is investigated by the energy calculations, which show that the route is competitive with that occurring via an H₃O⁺ ion. The vibrational frequencies calculated for a hydrated H₂OBr⁺ Br⁻ ion pair satisfactorily reproduce the set of RAIR spectroscopy experiments preformed at 180 K.