Raman Spectral Studies of the Effects of Solutes and Pressure on Water Structure

Raman spectra excited by 4800‐Å argon‐ion‐laser radiation and recorded photoelectrically have been obtained from a series of ternary aqueous solutions by back scattering and by 90° scattering using Cary and Jarrell–Ash double monochromators, respectively. The OD stretching region of HDO (and D₂O), 2300–2800 cm⁻¹, and the OH stretching region of H₂O plus HDO, ∼ 2800–3800 cm⁻¹, were examined for the following solutes: (1) NaPF₆, (2) NaSbF₆, (3) NaClO₄, (4) NaBF₄, (5) NaReO₄, (6) NaMnO₄ (infrared only), (7) NaIO₄, (8) NaNO₃, (9) NaClO₃, (10) NaSCN, (11) KI, (12) CsF, (13) Na₂SO₄, (14) (C₄H₉)₄NCl, and (15) Xe. (See Addendum for other solutes.) In addition, Raman spectra were obtained from HDO in H₂O at pressures from 1 to 3790 bar at 25°C, and from a $\text{5.0M}$ solution of NaClO₄ in HDO and H₂O at pressures of 690 and 3450 bar at 25°C. Several pronounced intensity maxima were observed in the OD and OH stretching regions for salts of extremely strong acids, (1)–(4), and some qualitative correlations with the strengths of the corresponding acids were found for (1)–(10). Solutes (1)–(11) were also found to act as structure breakers in water, but (12) was found to enhance structure, and some hydrogen bonding was found for (13). Further, no significant spectral effects were noted for (15) when the Xe concentration of the molten clathrate hydrate was $\text{0.8M}$ at 70°C and 1720 bar. For (14), however, an intense low‐frequency OD stretching component was observed near 2483 cm⁻¹. The (C₄H₉)₄N⁺ cation is the first found to produce pronounced effects in the OD and OH stretching regions, and the 2483 cm⁻¹ OD stretching component appears to arise from a marked structure‐making effect. The effect of pressure on the OD stretching contour of HDO in H₂O was observed to be small. Only a slight decrease in the frequency of the nonhydrogen‐bonded component centered near 2660 cm⁻¹ was uncovered to pressures of 3790 bar, and no significant pressure effect was observed for the NaClO₄ solution to 3450 bar. The high‐pressure spectra indicate that pressure is not very effective in breaking the hydrogen bonds in water, and the filling of holes as the result of a redistribution of angles and distances in the hydrogen‐bonded network is suggested as a more important effect.