Theoretical study on the ionic dissociation of halosulfonic acids in small water clusters

Ionic dissociations of the halosulfonic acids HSO3X, X = F, Cl, Br in an aqueous environment are studied by density functional theory and ab initio method calculations of the water clusters HSO3X–(H2O)n, n = 1–4. With a minimum of n = 3, HSO3X is dissociated as a result of proton transfer from HSO3X to H2O. Two competing stable configurations are reported: one with the halogen pointing away from water and the other with the halogen participating in hydrogen bonding with water. In the former, the S–X bond is stable with the successive addition of water molecules while in the latter the bond length increases gradually.