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

Details of the H3O+ and H2O structure in the first solvation shell about Cl- in aqueous HCl solutions are reported from x-ray absorption fine structure (XAFS) measurements. Results show increasing degrees 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 (concentrated acid). 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 than 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 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. The results further our understanding of the interaction H3O+ with Cl- that is of interest to fundamental physical chemistry and that has consequences in biochemical, geochemical and atmospheric processes.