Raman Spectra of Water in Concentrated Ionic Solutions

Abstract

Photoelectric Raman spectra of solutions of the four potassium halides and of Be2+, Mg2+, Cu2+, Zn2+, Sn2+, Al3+, La3+, and Pr3+ chlorides in ordinary water and in 5 mole % deuterated water show, in the region of the water stretching fundamentals, single broad bands similar to those of the pure solvents. Peak frequency shifts between solutions are small, about 1% or of the order of the available resolution, and are insensitive to the cation, except in the ZnCl2 solutions. Intensity and bandwidth changes are large, in the range 20%−100%, but these also correlate with the halide ion in the order F−<H2O<Cl−<Br−<I−. Direct cation effects are absent, even in solutions containing colored Cu2+ and Pr3+ ions near resonance with the 4358-Å exciting light, but in all of the solutions containing polyvalent cations, the molar intensity effect of Cl− is less than it is in KCl. The observed intensities can be related to anion effects when the Raman intensity behavior in solution is interpreted in terms of contribution by both components to the intermediate state of the scattering process, if complex formation is a common occurrence between chloride and polyvalent cations. Despite the strong excited-state interaction underlying intensity behavior, there is no indication that ion—water association leads to the large changes in frequencies or bandwidths of the water stretching fundamentals which are implied by a rigid hydration model of the ion in solution.