How Ions Affect the Structure of Water: A Combined Raman Spectroscopy and Multivariate Curve Resolution Study

Raman spectroscopy in combination with multivariate curve resolution (Raman-MCR) is used to explore the interaction between water and various kosmotropic and chaotropic anions. Raman-MCR of aqueous Na-salt (NaI, NaBr, NaNO3, Na2SO4, and Na3PO4) solutions provides solute-correlated Raman spectra (SC-spectra) of water. The SC-spectra predominantly bear the vibrational characteristics of water in the hydration shell of anions, because Na+-cation has negligible effect on the OH stretch band of water. The SC-spectra for the chaotropic I–, Br–, and NO3 – anions and even for the kosmotropic SO4 2– anion resemble the Raman spectrum of isotopically diluted water (H2O/D2O = 1/19; v/v) whose OH stretch band is largely comprised by the response of vibrationally decoupled OH oscillators. On the other hand, the SC-spectrum for the kosmotropic PO4 3– anion is quite similar to the Raman spectrum of H2O (bulk). Comparison of the peak positions of SC-spectra and the Raman spectrum of isotopically diluted water suggests that the hydrogen bond strength of water in the hydration shell of SO4 2– is comparable to that of the isotopically diluted water, but that in the hydration shell of I–, Br–, and NO3 – anions is weaker than that of the latter. Analysis of integrated area of component bands of the SC-spectra reveals ∼80% reduction of the delocalization of vibrational modes (intermolecular coupling and Fermi resonance) of water in the hydration shell of I–, Br–, NO3 –, and SO4 2– anions. In the case of trivalent PO4 3–, the vibrational delocalization is presumably reduced and the corresponding decrease in spectral response at ∼3250 cm–1 is compensated by the increased signal of strongly hydrogen bonded (but decoupled) water species in the hydration shell. The peak area-averaged wavenumber of the SC-spectrum increases as PO4 3– < SO4 2– < NO3 – < Br– < I– and indeed suggests strong hydrogen bonding of water in the hydration shell of PO4 3– anion.