Diffraction pattern and structure of aqueous hydrochloric acid solutions at 20 °C

Abstract

Hydrochloric acid solutions have been studied at concentrations from pure water to saturation with both x rays (HCl/H₂O) and thermal neutrons (DCl/D₂O). Such an extensive study reduces greatly the nonuniqueness of interpretation usually associated with diffraction data from liquids. The structure of D₂O molecules in liquid water is very similar to that found in the vapor phase, the OD distance of 0.955(5) Å being identical to the equilibrium distance in the vapor molecules. The existence of D₃O⁺ ions with an OD distance of 1.017(5) Å in concentrated DCl is firmly established. The D₃O⁺(H₃O⁺) ions are hydrogen bonded to four water molecules with an H₃O⁺⋅⋅⋅OH₂ distance of 2.52(2) Å and a D₃O⁺⋅⋅⋅D distance of 1.61(2) Å. The Cl⁻ ions appear to be four‐coordinated by oxygen atoms at a mean distance of 3.13(1) Å, but no pronounced Cl⋅⋅⋅D distances indicating a well‐oriented hydration shell about Cl⁻ could be identified.