Water in the Hydration Shell of Halide Ions Has Significantly Reduced Fermi Resonance and Moderately Enhanced Raman Cross Section in the OH Stretch Regions

Abstract

Water in the presence of electrolytes plays an important role in biological and industrial processes. The properties of water, such as the intermolecular coupling, Fermi resonance (FR), hydrogen-bonding, and Raman cross section were investigated by measuring the Raman spectra in the OD and OH stretch regions in presence of alkali halides (NaX; X = F, Cl, Br, I). It is observed that the changes in spectral characteristics by the addition of NaX in D2O are similar to those obtained by the addition of H2O in D2O. The spectral width decreases significantly by the addition of NaX in D2O (H2O) than that in the isotopically diluted water. Quantitative estimation, on the basis of integrated Raman intensity, revealed that the relative Raman cross section, σ(H)/σ(b) (σ(H) and σ(b) are the average Raman cross section of water in the first hydration shell of X(-) and in bulk, respectively), in D2O and H2O is higher than those in the respective isotopically diluted water. These results suggest that water in the hydration shell has reduced FR and intermolecular coupling compared to those in bulk. In the isotopically diluted water, the relative Raman cross section increases with increase in size of the halide ions (σ(H)/σ(b) = 0.6, 1.1, 1.5, and 1.9 for F(-), Cl(-), Br(-), and I(-), respectively), which is assignable to the enhancement of Raman cross section by charge transfer from halide ions to the hydrating water. Nevertheless, the experimentally determined σ(H)/σ(b) is lower than the calculated values obtained on the basis of the energy of the charge transfer state of water. The weak enhancement of σ(H)/σ(b) signifies that the charge transfer transition in the hydration shell of halide ions causes little change in the OD (OH) bond lengths of hydrating water.

Absolute proton hydration free energy, surface potential of water, and redox potential of the hydrogen electrode from first principles: QM/MM MD free-energy simulations of sodium and potassium hydration

“…These observations indicate that the Coulombic ion-solvent interactions are overestimated in the MM case, which does not take the reduction of the effective ion charge by CT effects into account. 58,104,107,180,[277][278][279][280][281][282][283] Furthermore, the strong correlation between the position and height of the first RDF peak when varying the parameters of a simple ion-solvent LennardJones interaction potential also suggests that it is impossible to adjust independently the two observables when using such a simple potential. In this respect, it is interesting to note that three recent approaches for improving the description of ion-water interactions at the MM level rely in effect on a weakening of the ion-solvent Coulombic interactions.…”

“…The importance of CT in ionic hydration has been underlined previously 58,104,107,180,[277][278][279][280][281][282] and is supported by experimental evidence. 283 To highlight this effect in the present simulations, the Mulliken charge 217,218 of the ion was calculated along the QM/MM MD simulations of Na + and K + . The corresponding time series and distributions are shown in Figs.…”

Ratiometric detection of Raman hydration shell spectra

“…For example, the ratio σ hydration /σ water was determined to be 0.6, 1.1, 1.5, and 1.9 for F À , Cl À , Br À , and I À respectively. [23] Here, the ratio also need to be calculated. The total water molecules in the aqueous solution can be calculated from water molecules in pure water with the same volume.…”

“…The value for Cl À agreed well with recent result. [23] For aqueous acetone-D6 solution (mole fraction 0.01), the ratio σ hydration / σ water was determined to 0.96 ± 0.02.…”

“…[17][18][19] Using these methods, the information of vibrational or rotational relaxation dynamics of water molecules in hydration shell is acquired. On the other hand, the statistical structure has been widely studied through MD simulations, [17,[19][20][21][22] Raman and IR spectroscopy [23][24][25][26][27][28] , X-ray and neutron diffraction. [29][30][31][32][33] Employing these methods, the hydrogen bond length and angles, the hydration number, and the spatial distribution of water molecules in hydration-shell are obtained.…”