Acid ionization of HBr in a small water cluster

The acid ionization by which HBr transfers a proton to a water molecule is studied for HBr in small water clusters as a first stage in a study of the corresponding Arctic atmospheric process. Electronic structure calculations , involving relatively large basis sets and including electron correlation effects, indicate that HBr is acid-ionized in a cluster of four water molec ules. The most stable structure in the general HBr(H2O)(4) system is a diam ond-like (trigonal bipyramidal) structure in which the bromide and hydroniu m ion species are bridged by three water molecules, and infrared spectral a nd ionization potential characteristics potentially useful for experimental study are provided. This structure is calculated to remain the thermodynam ically most stable one up to about 180 K, where HBr + (H2O)(4) becomes more stable. Aspects of the formation of the acid-ionized cluster from both HBr + (H2O)(4) and H2O + HBr(H2O)(3) are discussed. Possible implications for HBr acid ionization at the surface of ice, believed to be important for Arc tic atmosphere ozone depletion mechanisms, are briefly discussed.