Theoretical investigation of ionic dissociations of fluorosulfonic acid in microsolvated clusters

Ionization or ionic dissociation of fluorosulfonic acid (HSO3F) in the clusters HSO3F-(H2O)n (n = 1-3) and HSO3F-NH3-(H2O)n (n = 0, 1) is investigated by density functional theory and ab initio molecular orbital theory. The equilibrium structures, binding energies, and harmonic frequencies are calculated using the hybrid density functional (B3LYP) method with the 6-311++G** basis set and the second-order Møller-Plesset approximation (MP2) method with the 6-311++G** basis set. Harmonic frequencies are obtained from the B3LYP/6-311++G** calculations. Fluorosulfonic acid is found to require a minimum of three water molecules for ionization to occur and at least one water molecule to protonate ammonia. The corresponding clusters with fewer water molecules are found to be strongly hydrogen-bonded. The acid strength and related properties of fluorosulfonic acid are discussed and compared to those of perchloric acid and sulfuric acid in the context of clusters with ammonia and water.