Coupling of H2O librations and XO3 bending modes in barium and strontium chlorate, bromate and iodate monohydrates

Abstract

The i.r. and Raman spectra including Raman single crystal measurements of the isostructural series Ba(ClO3)2 · H2O, Ba(BrO3)2 · H2O, Sr(BrO3)2 · H2O and Sr(IO3)2 · H2O and of deuterated samples are presented for the range 200–700 cm−1 at 90 and 295 K and assigned to the unit cell group modes of both H2O librational and XO3 bending vibrations. The spectra reveal strong mixing of the H2O, HDO and D2O librational modes (Rr,Rt, Ry) with the XO3 bending vibrations ν2 and ν4. This coupling, which can be used for assignment of the librational modes, causes intensity transfers and frequency shifts up to 30 cm−1 compared to the unmixed modes. The strong temperature dependence of halfwidth and intensity of the H2O librational modes is probably due to orientational disorder of the water molecules. The intensity of the H2O twisting modes in both the Raman and i.r. spectra are discussed in terms of site symmetry (C2 or C1 in the case of orientational disorder) and bonding structure of the water molecules.