Abstract
Electrolyte solutions often show characteristic spectral features in their vibrational spectra, and a quantitative analysis of them is a key toward better understanding of the solvation structures and intermolecular interactions based on those spectral features. Here, such an analysis is carried out for the C≡N stretching mode of some electrolyte solutions of acetonitrile. It is found that a negative noncoincidence effect, in which the isotropic Raman component is located at a higher frequency than the anisotropic component, is observed for the band that newly appears upon solvation of salt, and based on a theoretical analysis that refers to quantum chemical calculations and molecular dynamics simulation results, it is shown that this phenomenon arises from the vibrational coupling among the C≡N bonds clustering around each metal ion (Li(+), Mg(2+), Ca(2+), or Zn(2+)). The transition dipole coupling mechanism reasonably well explains the sign and magnitude of this intermolecular vibrational coupling. The overall high-frequency shift of this newly appearing band (compared to the band of neat liquid) is largely explained by an electrostatic interaction model that takes into account the spatially nonuniform nature of the electrostatic potentials and fields operating from each metal ion on the solvent molecules around it. On the basis of these results, the relation between the spectral features and the solvation structures is discussed.