Fingerprint region of the spectrum

A further aid to identification comes from the fact that a molecule commonly has complex series of absorptions spanning a range of wavelengths. This fingerprint region of the spectrum may be too difficult to analyze in detail, but its presence enables us to recognize the substance by comparing the spectrum to an atlas of spectra. 

The C—O stretching band is strong and appears in the fingerprint region of the spectrum. The position is somewhat dependent on the physical state of the sample but it is usually possible to ascertain the type of hydroxyl compound under investigation thus m-cresol shows absorption in the phenolic C—O stretching region at 1330 cm " whereas the band at 1060 cm 1 in the spectrum of heptan-l-ol is characteristic for primary alcohols. 

It is highly unlikely that the IR spectra of two different compounds (except enantiomers) will show the same frequencies for all their various complex vibrations. For this reason, the infrared spectrum provides a fingerprint of a molecule. In fact, the region of the IR spectrum containing most of these complex vibrations (600 to 1400 cm-1) is commonly called the fingerprint region of the spectrum. 

Infrared spectroscopy can be applied to V. the characterization of polymeric materials at various levels of sophistication. As most commonly used, it is a rapid and easy method for the qualitative identification of major components through the use of group frequencies and distinctive patterns in the fingerprint region of the spectrum. Let s look at a couple of examples. 

For fibres of the same polymer from different manufacturers the fingerprint region of the spectrum may often differ, if only subtly in some cases. Figure 2.4 shows a comparison of the spectra of the nylon fibres from Williams carpet with those from a different manufacturer. As can be seen there are sufficient differences to allow a distinction to be made between them. 

Chapter 9 is concerned with functional groups that may contain —N=0 bonds or Si, P, S or the halogen atoms. With the exception of the X—H stretches derived from these elements and the nitroso (—N=0) and nitrite groups (—O—N=0), the large majority of the group frequencies associated with these elements fall below 1500 cm in the fingerprint region of the spectrum (I). 

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