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 wat er (D2O), methanol-d(4) (MeOD), and N,N-dimethylformamide-d(7) (DMF), respectively. A first-order difference method with chlorine isotopes s ubstitution was used to derive the Cl--dependent partial structure fac tors and radial distribution functions. The oscillation patterns of al l Cl--related structure factors normalized by the concentration persis t 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 f unctions 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 th e solvation number for Cl- in the methanol solutions were determined a s 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 solutio ns, 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 arou nd the chloride ion are proposed and discussed on the basis of the sol vent properties. (C) 1995 American Institute of Physics.