Neutron Diffraction Studies on the Hydrogen-Bonded Structure in Concentrated Aqueous Hydrochloric Acid Solutions

The neutron diffraction measurement with the H/D isotopic substitution technique has been carried out on the aqueous 21 mol% hydrochloric acid solutions, in order to investigate the short-range structure in the concentrated aqueous acid solution, in comparison with the structural information available on liquid pure water. Observed partial structure factors : aHH(Q), aXH(Q), and aXX(Q) (X: O, Cl), determined by the least-squares fitting analysis, have exhibited that the nearest-neighbor intermolecular hydrogen-bond distances, r(H···H) and r(O···H), between oxonium ion and water molecule in the 21 mol% hydrochloric acid solution were 2.02 ± 0.05 and 1.69 ± 0.02 Å, respectively; such values are much smaller than those in liquid pure water. Consequently, a strong intermolecular hydrogen bond between oxonium ion and its nearest-neighbor water molecules is concluded to be present in the concentrated aqueous hydrochloric acid solution.