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 ac id 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 structur e factors: a(HH)(Q), a(XH)(Q), and a(XX)(Q) (X: O, Cl), determined by the least-squares fitting analysis, have exhibited that the nearest-ne ighbor 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 Angstrom, respectively ; such values are much smaller than those in liquid pure water. Conseq uently, 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.