Raman Spectral Studies of the Effects of Perchlorate Ion on Water Structure

Abstract

Raman contours corresponding to the OD and OH stretching vibrations from HDO, as well as from H2O and D2O, were obtained at 25°C from a series of ternary aqueous solutions containing HDO, ClO4− (Li+, Na+, K+), and H2O or D2O. The Raman spectra were obtained photoelectrically using 4880-Å argon-ion-laser excitation, as well as conventional 4358-Å mercury excitation. Quantitative Raman data were obtained from ternary solutions having NaClO4 concentrations from 1 to 4M, and stoichiometric concentrations of 5.51M D2O (H2O as solvent), or 5.53M H2O (D2O as solvent). Raman spectra were also obtained from solutions having lower (and higher) HDO or NaClO4 concentrations. In addition, binary solutions of NaClO4 and H2O or D2O were examined. Addition of ClO4− to solutions containing HDO, and H2O or D2O produces a pronounced splitting of the OD and OH stretching contours from HDO, as well as from D2O and H2O. The splittings are directly observable in the Raman spectra. Two principal peaks or components, having frequencies nearly identical to components uncovered previously in the absence of ClO4−, result in the HDO case (or four major components in the D2O and H2O cases). The Raman intensities of the high-frequency OD or OH components from HDO (or of the corresponding high-frequency D2O or H2O components) increase relative to the low-frequency components with addition of ClO4−. The effects are evident even at 0.5M ClO4− and below. In addition, isosbestic frequencies are observed from the binary and ternary perchlorate solutions for bands of HDO, D2O, and H2O, and the values compare favorably with isosbestic frequencies obtained previously at a series of temperatures. Examination of the OD and OH stretching contours from HDO at high ClO4− concentration, at which the high-frequency components are nearly isolated, also indicates that the component shapes are Gaussian. The Raman data indicate that the ClO4− ion is a strong structure breaker, and that it does not hydrate appreciably, in agreement with the recent infrared absorbance results of Kecki et al. The data also provide clear evidence in contradiction to spectroscopic continuum models of water and aqeuous solutions, but mixture models involving several major classes of interactions, e.g., hydrogen-bonded or non-hydrogen-bonded OD or OH groups, and OD or OH groups engaged in the hydration of cationic and anionic species, but excluding ClO4−, are in accord with the data.