Structures and stability of hydrated clusters of hydrogen chloride, $HCl(H2O)n,$ $n=1–5$

The molecular structures of the hydrated clusters of the HCl molecule, $HCl(H2O)n,$ $n=1–5,$ are examined by employing density functional molecular orbital methods. The most stable structures of the $n=1–3$ clusters are found to be of the proton nontransferred type. In the case of the $n=4$ cluster, the proton nontransferred and proton transferred structures have nearly similar energies. There are several stable isomers for the $n=5$ case and the structures of these isomers are found to be all proton transferred. The relative stabilities of the direct ion-pair $H+Cl−(H2O)n$ and the indirect ion-pair $H3O+(H2O)n−1Cl−$ are discussed in conjunction with their structures. The prediction of the IR spectra of the stable $HCl(H2O)n$ clusters clearly indicate the large red-shifts of the H–Cl stretching and hydrogen-bonded O–H stretching frequencies.