Journal of Chemical Physics, Vol.103, No.18, 8174-8178, 1995
Neutron-Diffraction Study on Chloride-Ion Solvation in Water, Methanol, and N,N-Dimethylformamide
Pulsed neutron diffraction measurements have been carried out on 8.6, 5.8, and 1.7 molar lithium chloride (LiCl) solutions in deuterated water (D2O), methanol-d(4) (MeOD), and N,N-dimethylformamide-d(7) (DMF), respectively. A first-order difference method with chlorine isotopes substitution was used to derive the Cl--dependent partial structure factors and radial distribution functions. The oscillation patterns of all Cl--related structure factors normalized by the concentration persist up to the high momentum transfer region (similar to 10 Angstrom(-1)), suggesting the presence of the short-range ordering around chloride ion in the three solvent systems. The normalized radial distribution functions have revealed that methanol molecules are hydrogen bonded to a chloride ion with almost linear orientation of Cl ... D-O, as in the case of chloride hydration. The nearest-neighbor Cl-D distance and the solvation number for Cl- in the methanol solutions were determined as 2.21+/-0.03 Angstrom and 3.6+/-0.5, respectively, compared with 2.29+/-0.01 Angstrom and 5.8+/-0.5 for the aqueous solutions. The smaller solvation number for Cl- in the methanol solutions suggests that an Li+-Cl- ion association takes place in the solutions. In the DMF solutions, the first peak was observed at a much longer distance, similar to 2.85 Angstrom, and assigned to the distance between Cl- and the formyl H atoms of the DMF molecule due mostly to the ion-dipole interaction. The number of DMF molecules around the chloride ion was estimated as 6.8+/-0.5. The most likely conformations of the solvent molecules around the chloride ion are proposed and discussed on the basis of the solvent properties.
Keywords:X-RAY-DIFFRACTION;AQUEOUS-SOLUTIONS;SINGLE IONS;HYDRATION;THERMODYNAMICS;ASSUMPTION;ALCOHOLS;SOLVENTS