Variation of the Uncoupled OD Stretching Frequency with Electrolyte Concentration in Aqueous Electrolyte Solutions

Uncoupled OD stretching Raman spectra were measured for many aqueous electrolyte solutions as a function of electrolyte concentration. There are three types of variations of frequencies of uncoupled OD stretching Raman spectra with electrolyte concentration. The first type shows monotonous increase in frequency with increasing salt concentration, the second shows curved variation with a maximum higher than the frequency of pure liquid water, and the third exhibits a constant decrease in frequency with salt concentration. It is confirmed that the cationic effect on the OH (or OD) stretching frequency is very small so far as the charge of the cations is the same. Structure-making anions strengthen the average strength of hydrogen bonds and structure-breaking ones weaken the hydrogen bonds as compared with those in pure liquid water.