Structure of hydronium (H₃o⁺)/chloride (Cl ^-) contact ion pairs in aqueous hydrochloric acid solution: A zundel-like local configuration

A comprehensive analysis of the H₃O⁺ and H ₂O structure in the first solvation shell about Cl^- in aqueous HCl solutions is reported from X-ray absorption fine structure (XAFS) measurements. Results show increasing degree of contact ion pairing between Cl^- and H₃O⁺ as the HCl concentration increases from 6.0 m, 10.0 m, and finally 16.1 m HCl (acid concentrations are expressed as molality or mole HCl/1000 g water). At the highest acid concentration there are on average, approximately 1.6 H₃O⁺ ions and 4.2 H ₂O's in the first shell about Cl^-. The structure of the Cl^-/H₃O⁺ contact ion pair is distinctly different from that of the H₂O structure about Cl^-. The Cl-O bond length (2.98 Å) for Cl^-/H₃O⁺ is approximately 0.16 Å shorter than the Cl^-/H₂O bond. The bridging proton resides at an intermediate position between Cl and O at 1.60 Å from the Cl^- and approximately 1.37 Å from the O of the H₃O⁺. The bridging-proton structure of this contact ion pair, (Cl-H-OH₂), is similar to the structure of the water Zundel ion, (H₂O-H-OH₂⁺). In both cases there is a shortened Cl-O or O-O bond, and the intervening proton bond distances are substantially longer than for the covalent bonds of either HCl or H ₂O. A detailed structural analysis of the aqueous chloride species, Cl^-/(H₂O)_n, was also completed as part of this study in order to understand the relative importance of various XAFS photoelectron scattering paths. For aqueous Cl^- the measured Cl-O and Cl-H distances of 3.14 Å and 2.23 Å, respectively, are in excellent agreement with earlier neutron and X-ray diffraction results. Overall, these results significantly improve our understanding of the interaction of H ₃O⁺ with Cl^-. The results are of interest to fundamental physical chemistry and they have important consequences in biochemical, geochemical, and atmospheric processes.