Size-Dependent Formation of an Ion Pair in HSO4-(H2O) n: A Molecular Model for Probing the Microsolvation of Acid Dissociation.

With a p Ka of 2.0, HSO4- is not a strong acid as its dissociation percentage is only ∼10% in a solution of 1 M. However, our ab initio molecular dynamics and density functional theory calculations show that acid dissociation in the hydrate clusters, HSO4-(H2O) n, is quite facile at moderate sizes. It starts at around n = 12 and is completed by n = 16 when the energetics becomes very favorable. The dissociation explains the significant broadening at n = 16 in the S═O stretching region of the previously reported infrared photodissociation spectra for HSO4-(H2O) n as the solvation shell is tightened around the sulfate dianion and the proton. More importantly, HSO4-(H2O) n should provide an ideal model to probe the molecular details involved in an acid dissociation process.