Infrared spectroscopy of electrolyte solutions

Another way of assessing cation-solvent interactions is to examine the vibrational characteristics of the bond formed between the cation and the negative end of the solvent molecule s dipole. This bond is mainly electrostatic in character and therefore is much weaker than intramolecular covalent bonds. As a result the infrared band corresponding to vibration of the metal-solvent bond is normally observed in the far infrared at frequencies less than 500 cm. Some results obtained for the alkali metal cations in four aprotic solvents are summarized in table 5.8. 

It is clear that the frequency of the metal-solvent band decreases with increase in the size of the alkali metal, that is, with decrease in the field due to the charge on the ion. When ion pairing is absent, the value of the frequency is independent of the nature of the anion. This is definitely the case in DMSO, which strongly solvates cations because of its high donor number. In addition, the relatively high permittivity of this solvent reduces the influence of long-range ion-ion interactions. 

Because instrumentation capable of working in the far infrared is not commonly available, studies of interactions between polar solvent molecules and ions have more commonly involved intramolecular vibrational modes in the solvent 

By following the concentration dependence of the negative-going band at 2253 cm , one can determine the average number of AcN molecules which are coordinated to the metal ion in solution. Experiments show that the integrated intensity of this band is linear in the electrolyte concentration over reasonable concentration ranges. Interpretation of the negative slope of these plots requires that the extinction coefficient of free acetonitrile molecules be determined in a 

Cations also affect other solvent bands for AcN including the combination band at 2293 cm (V3 -I-V4), the C —C stretching mode at 918 cm (V4), and the C —C=N deformation mode at 746 cm (2vg). Changes in the intensity of these bands with electrolyte concentration may also be used to assess ion pairing in acetonitrile solutions [35, 36]. 

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