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 molecules. The most stable structure in the general HBr(H2O)4 system is a diamond‐like (trigonal bipyramidal) structure in which the bromide and hydronium ion species are bridged by three water molecules, and infrared spectral and ionization potential characteristics potentially useful for experimental study are provided. This structure is calculated to remain the thermodynamically 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 Arctic atmosphere ozone depletion mechanisms, are briefly discussed.