Applications of Infrared Spectroscopy to Structure Determination

Infrared spectra are very useful both for identification of specific organic compounds, and for determining types of compounds. For example, Figure 9-13 shows the infrared spectrum of a substance, C4H6O2, for which we wish to determine the compound type and, if possible, the specific structure. The most informative infrared absorptions for determining the compound type are between 1500cm−1 and 3600cm−1. Two groups of bands in this region can be seen at about 1700cm−1(s) and 3000cm−1, where (s) means strong; if we used (m) it would mean medium, and (w)(w) would mean weak. From Table 9-2 we can see that these bands are indicative of C=O (1700cm−1) and hydrogen-bonded OH of carboxylic acids (3000cm−1). The presumption is that there is a −CO2H group in the molecule, and we can derive some reassurance from the fact that the molecular formula C4H6O2 has enough oxygens to allow for this possibility.

Table 9-2 also shows that a −CO2H group should have a C−O absorption band between 1350cm−1 and 1400cm−1 and O−H absorption (bending frequency) between 1000cm−1 and 1410cm−1, and there is indeed a band of medium intensity at 1350cm−1 and a strong band at 1240cm−1. These absorptions, being in the fingerprint region, do not prove that the compound is a carboxylic acid; but if there were no absorptions in the 1000cm−1 to 1400cm−1 range, the presence of a −CO2H group would be highly questionable.

Figure 9-13: Infrared spectrum of a compound, C4H6O2

Tentatively, then, we may write a partial structure for C4H6O2 as

Figure 9-13. The alternative to a double bond would be a ring, which for C3H5 has to be a cyclopropyl ring. The structure that is most compatible with the spectrum is

Final identification may be possible by comparison with an authentic spectrum of cyclopropanecarboxylic acid, if it is available in one of the several standard compendia of infrared spectra. A total of about 150,000 infrared spectra are available for comparison purposes. You should check with the reference section of your library to see what atlases of spectral data are available to you.

The foregoing example illustrates the way structures can be determined from infrared spectral data. For many purposes, the infrared frequencies given in Table 9-2 are both approximate and incomplete. However, you could be easily frustrated in interpreting spectral data by being burdened with a very detailed table in which the unimportant is mixed with the important. The ability to use extensive tables effectively comes with experience. You should remember that tabulated infrared frequencies indicate only the range in which a given vibrational transition will fall. The exact value for a particular compound usually is meaningless because it will change depending on whether the spectrum is taken of the solid, liquid, or gaseous states, the solvent used, the concentration, and the temperature.

Remember that lower frequency means longer wavelengths and lower energy.