Neutron Diffraction Study of Aqueous Hydrochloric and Hydrobromic Acid Solutions

The structure factors Sm(Q) over a wide range of Q(1.7–25 Å−1) for aqueous solutions of DC1 and DBr (ca. 1 M) at room temperature have been determined by means of LINAC-TOF neutron diffraction. The intramolecular structures of D2O and D3O+ in the solutions are found to be identical with those in the “revised watery model” for water. Applying the subtraction method of analysis to diffraction data for the solutions, we have obtained the following results with respect to the structure of the nearest hydration shell around ions in the DCl and DBr solutions: (a) D3O+ is tetrahedrally coordinated with four water molecules and Cl− and Br− are octa-hedrally coordinated with six ones, (b) the O–O distances between the central oxygen atom and that of hydrated water molecules for D3O+ are 2.88±0.05 Å, (c) the anion-oxygen distances are 3.10±0.05 Å for Cl− and 3.21±0.05 Å for Br−, and (d) around cations (D3O+) water molecules take the configuration to orient the axis of one of the two lone-pair hybrids on a straight line joining an oxygen atom and the cation, while around anions (Cl− and Br−) those take the configuration to orient the vector which bisects the D–O–D angle on a straight line joining an oxygen atom and the cation.