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 t...