Structure of hydronium (H3O+)/chloride (Cl-) contact ion pairs in aqueous hydrochloric acid solution: a Zundel-like local configuration.

A comprehensive analysis of the H(3)O(+) and H(2)O 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 H(3)O(+) 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 H(3)O(+) ions and 4.2 H(2)O's in the first shell about Cl(-). The structure of the Cl(-)/H(3)O(+) contact ion pair is distinctly different from that of the H(2)O structure about Cl(-). The Cl-O bond length (2.98 A) for Cl(-)/H(3)O(+) is approximately 0.16 A shorter than the Cl(-)/H(2)O bond. The bridging proton resides at an intermediate position between Cl and O at 1.60 A from the Cl(-) and approximately 1.37 A from the O of the H(3)O(+). The bridging-proton structure of this contact ion pair, (Cl-H-OH(2)), is similar to the structure of the water Zundel ion, (H(2)O-H-OH(2)(+)). 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(2)O. A detailed structural analysis of the aqueous chloride species, Cl(-)/(H(2)O)(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 A and 2.23 A, 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(3)O(+) with Cl(-). The results are of interest to fundamental physical chemistry and they have important consequences in biochemical, geochemical, and atmospheric processes.