Single crystal Raman studies of the H 2O librational modes of isomorphic MX 2·H 2O (M = Ba, Sr; X = Br, I)

Abstract

The librational modes of the water molecules in the isomorphic MX 2·Y 2O (M=Sr, Ba; X=Br, I; Y=H, D) are assigned to the wagging (R γ), twisting (Rt), and rocking vibrations (Rr) by means of single crystal Raman technique. The order of the librations in R γ > Rt > Rr in energy and Rt > Rr in intesity (R γ not observable), as predicted for tetrahedrally coordinated water molecules in solids. The data obtained are discussed with respect to (i) the strength of the hydrogen bonds, (ii) repulsive effects of the lattice (owing to the different volume increments of the H 2O molecules), and (iii) metal oxygen interactions (shown by the different enthalpies of hydration). The H 2O twisting vibrations are only little affected by the hydrogen bonds present (opposite to the wagging and rocking modes), but very sensitive to both repulsive effects and metal oxygen interaction.