The Fingerprint Region and Infrared Spectroscopy
The Infrared region is part of the Electromagnetic Spectrum that is dividing up all types of electromagnetic radiation. This radiation is divided up rather arbitrarily into a number of regions based on their wavelengths: Gamma < 10 nanometers, Ultraviolet radiation, Visible light 0.4 to 0.7 micrometers, Infrared Radiation, Microwaves, Radio waves
In infrared spectroscopy, the infrared absorption bands between 1250 cm-1 and 675 cm-1 generally are associated with complex vibrational and rotational energy changes of the molecule as a whole and are quite characteristic of particular molecules. This part of the spectrum is often called the “fingerprint” region and is extremely useful for determining whether samples are chemically identical. The spectra of 2-propanone and 2-butanone are seen to be very similar in the region 4000 cm-1 to 1250 cm1 but quite different from 1250 cm-1 to 675 cm-1. The fingerprint region of the spectrum is individual enough so that if the infrared spectra of two samples are indistinguishable in the range of frequencies from 3600 cm-1 to 675 cm-1 it is highly probable that the two samples are of the same compound (or the same mixture of compounds).
Characteristic stretching and bending frequencies occur in the fingerprint region, but they are less useful for identifying functional groups, because they frequently overlap with other bands. This region is sufficiently complex that a complete analysis of the spectrum is seldom possible.
Alkanes and Cycloalkanes in Infrared Spectroscopy
The infrared spectra of the alkanes show clearly absorptions corresponding to the C—H stretching frequencies at 2850 cm-1 to 3000 cm-1. The C—C stretching absorptions have variable frequencies and are usually weak. Methyl (CH3—) and methylene (—CH2—) groups normally have characteristic C—H bending vibrations at 1400 cm-1 to 1470 cm-1. Methyl groups also show a weaker band near 1380 cm-1. Two sample infrared spectra that illustrate these features are given in Figure 9-11.
The infrared spectra of the cycloalkanes are similar to those of the alkanes, except that when there are no alkyl substituents the characteristic bending frequencies of methyl groups at 1380 cm-1 are absent.
Figure 9-11 Infrared spectra of (a) octane and (b) 2,2,4-trimethylpentane as pure liquids. Notice the C—H stretching around 2900 cm-1 and C—H bending frequency around 1460 cm-1. The bands near 1370 cm-1 for 2,2,4-trimethylpentane are characteristic of methyl C—H bending frequencies.
A moderately strong CH2 “scissoring” frequency is observed between 1440 cm-1 and 1470 cm-1 the position depending somewhat on the size of the ring. These features of the infrared spectra of cycloalkanes are illustrated in Figure 9-12 using cyclooctane and methylcyclohexane as examples.
Figure 9-12 Infrared spectra of (a) cyclooctane and (b) methylcyclo-hexane. These spectra can be compared profitably with those in Figure 9-11.
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, C4H602, 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 1500 cm-1 and 3600 cm-1. Two groups of bands in this region can be*seen at about 1700 cirrus) and 3000 cm-1(s), where (s) means strong; if we used (m) it would mean medium, and (w) would mean weak. From Table 9-2 we can see that these bands are indicative of C=0 (1700 cm-1) and hydrogen-bonded OH of carboxylic acids (3000 cm-1). The presumption is that there is a —C02H group in the molecule, and we can derive some reassurance from the fact that the molecular formula C4H602 has enough oxygens to allow for this possibility.
Table 9-2 also shows that a —C02H group should have a C—O absorption band between 1350 cm-1 and 1400 cm-1 and O—H absorption (bending frequency) between 1000 cm-1 and 1410 cm-1, and there is indeed a band of medium intensity at 1350 cm-1 and a strong band at 1240 cm-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 1000 cm-1 to 1400 cm-1 range, the presence of a —C02H group would be highly questionable.
Figure 9-13 Infrared spectrum of a compound, C4H602
A propyl group would be C3H7, and C3H5 has two hydrogens less, which indicates the presence of a double bond or a ring. However, Table 9-2 shows that a double bond should have an absorption of variable intensity at 1600 cm-1 to 1680 cm-1 and there is no clear sign of such an absorption in 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. To become familiar with infrared spectra, we strongly recommend that you work Exercises 9-10 and 9-11. An important tool of the organic chemist is Infrared Spectroscopy.