Hydrogen-Bonded Liquids

Abstract

Vibrational and collision-induced (CI) Raman spectra were obtained from liquid water, and from aqueous solutions of NaClO4 (7.4 M), NaI (8.2 M), and sucrose (2.3 M). The inter- and intramolecular vibrational spectra were found to lie above the collision-induced (CI) Raman baselines which were approximated, above 20 cm−1, by dlnI/dlnw = −m, with m ≈ 1.2–1.3 (I = intensity, w = vibrational frequency in cm−1). Significant vibrational effects above the CI baselines, e.g., shape changes in the restricted translational, librational, H2O bending, and OH-stretching bands, including marked changes in the depolarization ratios, were observed for the solutions, compared to pure water. The pure isotropic Raman intensities of the restricted translational band near 155 cm−1, and of the OH-stretching band near 3480 cm−1, were enhanced significantly for the NaI solution, compared to pure water. These modes, which refer to restricted translations of first-sphere H2O molecules against the I− ion, and vibrations of the proton along the I−⋯H−O hydrogen bond direction, respectively, both involve increases of the isotropic polarizability derivatives because of the high polarizability of the I− anion, compared to H2O. Increased Raman polarization and sharpening of the hydrogen bond O⋯H−O stretching band from the sucrose solution near ≈200 cm−1 was also observed, along with a prominent 3210 cm−1 OH-stretching component from the concentrated sucrose-water glass. These observations suggest an ordering of the hydrogen bonds via the sucrose structure. Increased Raman polarization in the restricted translational region from the NaClO4 solution indicates strong interactions between ClO4− and H2O, which are also evident from the large librational intensity enhancements. The perchlorate-water interactions may involve bifurcation between pairs of perchlorate oxygen atoms, and protons of first-sphere H2O molecules.