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 SO42- 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 PO43- 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 SO42- 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 similar to 80% reduction of the delocalization of vibrational modes (intermolecular coupling and Fermi resonance) of water in the hydration shell of I-, Br-, NO3-, and SO42- anions. In the case of trivalent PO43-, the vibrational delocalization is presumably reduced and the corresponding decrease in spectral response at similar to 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 PO43- < SO43- < NO3- < Br- < I- and indeed suggests strong hydrogen bonding of water in the hydration shell of PO43- anion.