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

With a p K of 2.0, HSO 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, HSO(HO) , 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 HSO(HO) as the solvation shell is tightened around the sulfate dianion and the proton. More importantly, HSO(HO) should provide an ideal model to probe the molecular details involved in an acid dissociation process.