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

With a pK(a) 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) as the solvation shell is tightened around the sulfate dianion and the proton. More importantly, HSO4-(H2O), should provide an ideal model to probe the molecular details involved in an acid dissociation process