Ab initio MP2 equilibrium geometries of the clusters (H2O)mHCl, m = 1-3, and (H2O)n, n = 1-4, are reported. It is found that the clusters form hydrogen-bonded rings, with HCl acting as a proton donor. There is a significant increase in the HCl bond length as the cluster size increases, as well as a large red-shift in the HCl stretching frequency. The dissociation energy of HCl from (H2O)nHCl is appreciably larger for the trimer and tetramer than for the dimer, which is a consequence of cooperativity effects in the hydrogen bonds. We discuss the relevance of our findings to the understanding of the adsorption of HCl on ice. © 1995 American Chemical Society.
