Theoretical study of the hydrogen chloride trihydrate

Abstract

AbstractStructural, energetic, and spectroscopic properties of the hydrogen chloride trihydrate (H2O)3HCl have been calculated by ab initio and density functional methods. The stationary points on the potential energy surface and the harmonic frequencies were obtained with the density functional theory, the second‐order Møller–Plesset theory, and the self‐consistent reaction field theory, while the corresponding interaction energies and binding energies were computed using the coupled‐cluster method restricted to single, double, and noniterative triple excitations. The origins of the bonding have been characterized by symmetry‐adapted perturbation theory of pair and three‐body interactions. The global minimum on the potential energy surface of the cluster corresponds to a cyclic structure with the water and hydrogen chloride molecules connected by hydrogen bonds. Another minimum, corresponding to an ionic form of the trihydrate, (H2O)2H3O+Cl−, has a binding energy only ≈2 kcal/mol higher than the global minimum, but it is separated by a large barrier. The pathway connecting the two minima has been found, and the structure of the corresponding transition state has been determined. The geometry of the ionic minimum agrees well with the crystal structure of the hydrogen chloride trihydrate from single‐crystal X‐ray diffraction studies. Also, the computed harmonic frequencies and their theoretical assignment are in agreement with the experimental data recorded in the gas phase in argon matrices and in the crystalline phase. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002