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 H2O 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 H2O'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 angstrom) for Cl-/H3O+ is approximately 0.16 angstrom shorter than the Cl-/H2O bond. The bridging proton resides at an intermediate position between Cl and O at 1.60 angstrom from the Cl- and approximately 1.37 angstrom 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 CI-O or O-O bond, and the intervening proton bond distances are substantially longer than for the covalent bonds of either HCI or H20. 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 angstrom and 2.23 angstrom, respectively, are in excellent agreement with earlier neutron and X-ray diffraction results. Overall, these results significantly improve our understanding of the interaction of H3O+ with Cl-. The results are of interest to fundamental physical chemistry and they have important consequences in biochemical, geochemical, and atmospheric processes.