Low frequency raman investigation of water and aqueous solutions

Abstract

The Raman spectra of water and aqueous salt solutions are presented in the form of a vibrational density of states. R(ω). This R(ω) format reveals with greater definition the low frequency Raman bands due to polarizability changes in the weak bonds of intermolecular complexes. For pure water at 25ﾟC, broad bands were observed at 66 cm⁻¹ and 192 cm⁻¹ due to hydrogen bond bending and stretching modes. High signal to noise ratios achieved by multiple scans permitted the construction of difference spectra which greatly assisted the measurement of peak frequencies and depolarization data. The 192 cm⁻¹ band in the spectrum of water was found to be slightly polarized while the remainder of spectrum exhibited largely depolarized features. The hydrogen bond stretching mode of water shifted 6 cm⁻¹ in D₂O and 17 cm⁻¹ in H₂¹⁸O. This mode is interpreted as arising from oxygens moving about the hydrogen involved in the hydrogen bond but with the proton remaining closer to one of the oxygens. The effects of salts on the water spectrum have also been investigated. Most salts increase the relative intensity of the water spectrum although typical structure breakers cause a decrease in the relative intensity. New bands have also been observed when salts are added to water. Reorientation of the CO3²⁻ ion give rise to a depolarized scattering at 92 cm⁻¹ in aqueous solutions. Polarized bands due to cation and anion hydrates have also been observed. The symmetric stretch of the Mg(H₂O)₆²⁺ ion is observed at 359 cm⁻¹ in MgCl₂ while a polarized band for LICI(aq) occurs at 384 cm⁻¹. Polarized bands at 263 cm⁻¹ for saturated KF(aq). 298 cm⁻¹ for 10M NaOH(aq), 293 cm⁻¹ for 11.5M KOH(aq), 286 cm⁻¹ for 5.9M RbOH(aq) and in CsOH(aq) are assigned to stretching of the O-H∙∙∙X⁻ band.