Acid Dissociation in (HX)n(H2O)n Clusters (X = F, Cl, Br, I; N = 2, 3)

The interactions between two or three hydrogen halide molecules and the same number of water moieties are investigated through a systematic exploration of the corresponding potential energy surfaces using a stochastic methodology in conjunction with density functional theory computations. Our results indicate that HF, the weakest acid in the series, is partially dissociated. Similarly, HCl, HBr, and HI undergo dissociation in the presence of three, two, and two water molecules, respectively. The decrease in the number of water molecules required for dissociation, when compared with clusters with one single HX molecule, suggests cooperative effects. Interestingly, the hydrogen-bridged bihalide anions (XHX-) are present in the global minimum of (HX)n(H2O)n clusters with X = Br, I and n = 2, 3.