Temperature dependence of Raman band parameters of aquated cations

Abstract

The band parameters of the relatively diffuse, polarized, low frequency Raman lines, assigned to the symmetric stretching vibration of the aquated cations [In(H₂O)₆]³⁺ and [Hg(H₂O)₄]²⁺ have been measured over wide temperature and concentration ranges. The data, combined with the results of an earlier study of [Mg(H₂O)₆]²⁺ and [Zn(H₂O)₆]²⁺ and fewer data for [Cd(H₂O)₄]²⁺, provide insight into the structure, stability and lability of the primary hydration sphere. The molar intensities of the band increase in the order Mg²⁺ < Zn²⁺ < Cd²⁺ < Hg²⁺ < In³⁺ and are assumed to reflect the degree of covalence in the interaction. The data suggest that replacement of water by nitrate ion stabilizes the remaining coordinated water molecules, whereas chloride ion destabilizes these. Each nitrate ion which coordinates to Hg²⁺ replaces two water molecules. Increased temperature increases the degree of association between indium(III) or mercury(II) and nitrate ion.