Structure of Hydronium (H3O+)/Chloride (Cl−) Contact Ion Pairs in Aqueous Hydrochloric Acid Solution: A Zundel-like Local Configuration

摘要

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 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 H₂O. 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 H₃O+ with Cl−. The results are of interest to fundamental physical chemistry and they have important consequences in biochemical, geochemical, and atmospheric processes.