Infrared spectroscopy of non-electrolyte solutions

When a polar solute such as acetonitrile is dissolved in another organic solvent, the shift in the frequency of the polar group may be used to monitor solute-solvent interactions. This type of study is especially interesting because it demonstrates clearly the importance of short-range chemical interactions in determining the properties of the solution. 

Solvent-induced frequency shifts have also been studied for dimethylsulfoxide (DMSO), which is a strong Lewis base. Most solvents behave as Lewis acids and interact with DMSO via the electron density on the — S=0 group. This interaction leads to a red shift for this band which is as large as 56 cm in the presence of acetic acid [34]. The magnitude of the shift correlates well with the solvent s acceptor number giving the relationship 

For this system, removal of electron density from the — S=0 bond results in its being weakened as one would expect without knowing the detailed molecular orbital structure. 

Acetone provides another interesting system for studying intermolecular interactions. The basicity of this solvent, whose DN is 17.0, is close to that of water. On the other hand, it is a very weak Lewis acid with an AN equal to 12.5. Thus, one expects the C=0 stretching frequency V3 to shift with change in the acidity of the solvent in which acetone is dissolved. A good correlation is found between the frequency of this band in 22 solvents and the solvent s acceptor number AN, with the frequency shifting in the red direction as AN increases. The equation describing the correlation is 

The effects of intermolecular interactions are seen much more clearly for the Vjy mode which is the skeletal asymmetric stretch. Values of the frequency of this vibration are plotted against solvent AN in fig. 5.22. The equation describing the linear correlation is 

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