Halide-oxygen interactions in solutions. Part one. Chloride-oxygen interactions in aqueous hydrogen chloride

Abstract

The solvation of chloride ion in some concentrated aqueous hydrogen chloride solutions has been measured at three concentrations, 7.48, 10.08, and 12.28 N hydrochloric acid, by x-ray diffraction. In each case the primary solvation sphere, which contains approximately eight waters per chloride, as well as the secondary sphere about chloride are clearly discernible. The former is characterized by Cl−O interactions (hydrogen-bonded) at an average distance of ca. 3.25 A. In the secondary sphere, chloride ion is separated from each oxygen by ca. 6.2 A. The primary and, particularly, the secondary hydration spheres about chloride ion in these solutions are considerably more ordered than the corresponding hydration spheres in aqueous alkali metal chloride solutions. Because of the requisite sharing of water molecules by neighboring chloride ions, it is not possible to unambiguously determine the average primary sphere geometry of water molecules, about the chloride ions. No apparent correlation between the highly ordered primary and secondary solvation spheres about chloride and the various models proposed for the “structure” of liquid water are noted. Evidence for “short” O−O hydrogen-bonded contacts at 2.5–2.6 A is given, but no H−Cl and/or H−O contact distances are evidenced.