Structure of Chloric Acid Hydrates and Equilibrium Composition of the HClO4–H2O System

Composition, structure, energy parameters, and existence regions of hydrate complexes formed in the HClO₄–H₂O system are studied by IR spectroscopy and quantum chemical methods. Three concentration-structural regions are determined. The HClO₄–H₂O solutions diluted to the 1:13.3 molar ratio of components contain \(\text{ClO}^{-}_4\) and \(\text{H}_5\text{O}^{+}_2\) solvate-separated ions, each hydrated by four water molecules. More concentrated solutions (1:13.3-1:5) contain hydrated ion pairs \(\text{ClO}^{-}_4\cdot\text{H}_5\text{O}^{+}_2\) that are contact ion pairs if the [HClO₄]:[H₂O] ratio falls within the region 1:9-1:5. As the acid concentration increases further up to the transition of the HClO₄–H₂O system into the solid phase, pair complexes are formed. The structure of the latter is formed by a cycle of two contact ion pairs \(\text{ClO}^{-}_4\cdot\text{H}_5\text{O}^{+}_2\) connected by two H-bonds. The solid phase consists of interacting polymer chains formed by \(\text{ClO}^{-}_4\cdot\text{H}_5\text{O}^{+}_2\) and \(\text{ClO}^{-}_4\cdot\text{H}_3\text{O}^{+}\) ion pairs.