Infrared Spectroscopy of Polar Solvents

All polar solvents considered in chapter 4 are polyatomic species and therefore have vibrational spectra. Some vibrational modes are infrared active and others are Raman active. Especially interesting are those modes associated with the molecular dipole because they give information about intermolecular interactions. In this section, attention is focused on the infrared spectra of pure polar solvents. 

The most common protic solvent is water. It is also one of the most complex from the point of view of vibrational spectroscopy because of its highly structured nature. Since water is a triatomic, non-linear molecule it has three vibrational modes, which are illustrated in fig. 5.13. The Vj mode is the symmetrical stretch V2 is the bending mode and V3 is the asymmetrical stretch. All three vibrational modes for water are active in the infrared because they involve changes in the dipole moment. Activity in the Raman spectrum requires that the polarizability of the molecule changes during vibration. Analysis of this aspect of molecular properties is more difficult but it shows that all three modes are also Raman active. A summary of the frequencies of these vibrations for H2O, and the isotopes D2O, and HOD determined from gas phase spectra are given in table 5.7. 

The difficulties in interpreting the vibrational spectrum of water are obvious. Because of the broad nature of the bands, the two stretching modes Vj and V3 overlap. In addition, overtones complicate the assignment of bands. Thus, the first overtone of the V2 bending mode, 2v2, lies close in frequency to the stretching modes Vj and V3. For these reasons, there are advantages in studying HOD, a molecule for which the vibrational frequencies are quite different. As a result, 

Protic solvents always have more complex infrared spectra because of the presence of hydrogen bonding in the liquid state. In methanol, this involves interaction of the acidic proton on the OH group in one molecule with the oxygen atom in an adjacent molecule (fig. 5.15). The infrared spectrum shows a wide band centered at 3346 cm which is due to the -OH stretch. When methanol is dissolved as a dilute solute in carbon tetrachloride, this band is sharp and appears at 3644 cm . An -OH bending mode appears at 1449 cm. Another broad band due to -OH out-of-plane deformation is centered at 663 cm. The other features of the methanol spectrum are due to the vibrational modes of the CH3- group or to skeletal vibrations [27]. 

Another example of a protic solvent is formamide in which hydrogen bonding involves the proton in the -NH2 group and the carbonyl oxygen in an adjacent molecule (see fig. 5.16). A broad band centered at 3320 cm is due to the asym- 

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