The infrared spectrum provides the largest number of characteristic properties of a compound. It also serves as a powerful ‘analytical tool’ for the extensive and intensive study of molecular structure.
In fact, infrared absorption spectra are due to changes in vibrational energy accompanied by changes in rotational energy. Broadly speaking, the range in the electromagnetic spectrum that extends from 0.8 to 200 μ is referred to as the infrared region. In usual practice, however, either the wavelength (μ) or the wave number (cm–1) is employed to measure the position of a given infrared absorption. More precisely, the infra-red regions may be categorized into three distinct zones based on their respective wave numbers and wave-lengths as stated below :
Another school of thought advocates that there are two general regions in the infrared spectrum, namely : (a) Group frequency region : having a wavelength ranging from 2.5 to 8.0 μ and a wave number from 4000-1300 cm–1 ; (b) Fingerprint region : having a wavelength ranging from 8.0-2.5 μ and a wave number from 1300-400 cm–1.
Here, the stretching and bending vibrational bands associated with specific structural or functional groups are observed frequently.
Example : The C = O stretching frequency is about 1700 cm–1 ; whereas the C—H stretching frequency is about 3000 cm–1 and both of them are almost independent of the rest of the molecule as depicted in Table 22.1.
Here, the vibrational modes depend solely and strongly on the rest of the molecule.
Example : The C—C stretching frequency depends largely on what else is bonded to the carbon atoms.
It is interesting to observe here that this particular region of the spectrum is densely populated with bands. As we know that no two ‘fingerprints’ could be identical in human beings, exactly in a similar manner no two compounds may have the same ‘fingerprint region’. Thus, each and every molecule essen-tially gives rise to a unique spectrum which offers a characteristic feature of the same.