IR spectroscopy is a powerful tool for investigating the structure of aqueous systems.
Changesinvibrational frequencies and intensities of the absorption band provide information about the structure of the associated water molecules.
The water molecule has Czv symmetry when the intermolecular interaction is symmetrical to both OH bonds as in 2:l complexes.
In this case the frequency difference of the two stretching vibrations v3 and vl is nearly constant.
If the intermolecular interaction is unsymmetric to the OH bonds as in 1:l complexes the band separation of "3 and vl increases markedly in relation to the increase of the unsymmetry.
The IR overtone region is more suitable for the study of the structure of liquid water or aqueous solutions than the IR fundamental region.
The reason is the higher intensity of the absorption bands of the "free" OH vibration compared to the H-bonded OH groups.
The ratio of the intensities is inverse in the fundamental region. Furthermore it is possible to measure quantitatively in the overtone region and there
The results are estimations of the H-bonded aqueous systems.