Lattice vibration spectra. XXII—infrared and Raman spectra of strontium chloride dihydrate, SrCl₂·2H₂O and SrCl₂·2(H, D)₂O

Abstract

~ ~~The infrared absorption and Raman spectra of SrClz-2H20 powder as well as SrClZ.2Dz0, and partially deuterated and isotopically dilute samples, have been recorded in the range from 30 to 4000 cm-' at 95 and 295 K and analysed with regard to the strongly asymmetric force field in the H 2 0 molecules. This is shown by an unusually large splitting of the OH and OD stretching fundamentals of the HDO molecules: 3426 and 3243 cm-' and 2534 and 2408 cm-' respectively at 95 K. Uncoupled OD stretching frequencies of SrC12.6H20 and SrCI,-HzO are also presented. In SrClz.2H20 both a relatively strong and a weak hydrogen bond to chloride ions are present; these are essentially straight. The spectra of SrCIz-2Hz0 exhibit the following characteristics: (1) no intramolecular coupling of the OH stretching vibrations; (2) very large Fermi resonance of vOH and 2 & 4 (3) the same high intensity of HzO twisting librations in the infrared spectra as for wagging and rocking modes; (4) splitting of the librational fundamentals of the HDO molecules, as large as 100 cm-', caused by different structural arrangements of the H and D atoms in the asymmetric HzO molecules; and (5) strong intermolecular coupling of bending and librational fundamentals, which results in correlation splittings up to 75 cm-'. Possibilities of assigning the librational modes of HzO molecules which are in an asymmetric force field are discussed.

Profile structure refinement of SrCl2.2D2O by neutron powder diffraction

“…It can serve as a model system for studying solid hydrates with highly asymmetric force fields of the water molecules and isotope effects involved (Lutz, 1982(Lutz, , 1988. This is due to the presence of only one kind of strongly distorted H20 molecule in the structure (Lutz, Pobitschka, Christian & Becker, 1979). For such studies (Mrller, 1993) knowledge of the crystal structure including the true H-atom positions is necessary.…”

“…2. The OH and OD stretching modes recorded by IR and Raman spectroscopy of isotopically dilute samples (Lutz, Pobitschka, Christian & Becker, 1979) can now be undoubtedly assigned, viz. (2) is strongly bent as inferred from the small temperature shift of the respective OH and OD bands (Voo/dT < 0.03 cm-~ K-1).…”