Fingerprint frequencies below

Bands below 1350 cm may be either group frequencies or fingerprint frequencies. 

At frequencies with values greater than 1500 cm i it is generally possible to assign each absorption band in an infrared spectrum. This is not the case for most absorptions observed below 1500 cm. This region is referred to as the fingerprint region, since quite similar molecules give different absorption patterns at these frequencies. 

C—H stretching frequencies lie below 3000 cm for sp -hybridized carbon and above 3000 cm for sp. The fingerprint region is used less for determining structure than for verifying whether two compounds are identical or not. 

FTIR Studies of Polymers For the most part, the techniques used in the measme-ment of FTIR are the same as those applied to conventional samples. Spectral features of a component in a polymer are isolated from the solvent bands and from aU other compounds present in the system. Analysis is carried out by computer programs. Figure 17.26 shows the spectrum of polystyrene with an IR microscope through a 10-pm aperture. We see that the baseline is flat even below 1000 cm . This is very important because the frequency range of the IR spectrum below 1300 cm especially that below 1000 cm , is known as the fingerprint region. Changes in frequency and intensity may be used to determine the microscopic characterization of the polymer to see if there exists any defect due to stress or the environment. 

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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