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

Abstract

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.

Nature of hydration shells of a polyoxy‐anion with a large cationic centre: The case of iodate ion in water

“…The polyoxy‐anions form a major class of ions which have great relevance to various atmospheric and biological systems. These ions have multiple charge centers and their effects on the structure and dynamics of water have been the topic of many earlier studies in the literature …”

Effects of HPO42− and SCN− on the hydrogen bond network of water: femtosecond OHD‐RIKES and FTIR measurements

“…The band at 3290 cm −1 is the symmetric stretching vibration of tetrahedrally coordinated water molecules or ordering HB or ice‐like HB (also assigned as Fermi resonance), and the band at 3469 cm −1 is the symmetric stretching vibration with less coordinated water molecules or liquid‐like HB . These two stretching vibrations are both Raman and IR active, and then, the current FTIR measurement provides the same information as Raman spectroscopy does.…”

“…Raman or IR spectroscopy has been employed to indirectly study the effect of an ion on the intermolecular HB network of water in its aqueous solution through monitoring the oxygen–hydrogen (O–H) stretching vibration of water . The increase of the intensity or the red shift of the O–H stretching vibration of water in aqueous ionic solution means the HB ‘structure‐making’ character of an ion in water, and the decrease of the intensity or the blue shift of the O–H stretching vibration of water in aqueous ionic solution means the HB ‘structure‐breaking’ character of an ion in water . Besides indirectly exploring the ion effect on the intermolecular HB network through inspecting the change of the intramolecular mode in the presence of ion, several low‐frequency vibrational spectroscopic techniques have also been utilized to directly inspect the effect of an ion on the intermolecular HB network of water in its aqueous solution.…”

Raman spectroscopic investigation on pure D2O/H2O from 303 to 573 K: interpretation and implications for water structure

“…More commonly speaking, the much broader OH stretch band of liquid water than that of the gas water is due to an increase in the range of molecular environments . The broadening is inhomogeneous, which has been largely attributed to factors such as inhomogeneity of HB configurations, intermolecular vibrational couplings, and Fermi resonance (FR) . Many authors deconvoluted the OH stretch band into several components (commonly over three) that are assigned to water molecules engaged in different HB environments.…”

“…In a study on dilute HOD in liquid D 2 O, Auer et al decomposed the distribution of frequencies in the OH stretch region in terms of sub‐ensembles of HOD molecules with different local HB environments. On the other hand, some other authors stress the importance of intermolecular vibrational coupling and FR that are included in the analysis of vibrational spectroscopic data . For example, Ahmed et al decomposed the OH stretch band into three different Gaussian peaks that represent the contribution of intermolecular coupling/FR and HB interaction.…”