The Fingerprint Region 1500-600 cm

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. 

We have assumed so far that each band in an infrared spectrum can be assigned to either a particular deformation of the molecule, the movement of a group of atoms, or the bending or stretching of a particular bond. This is possible for many bands, particularly stretching vibrations of multiple bonds which are well behaved . However, many vibrations are not so well behaved and may vary by hundreds of wavenumbers, even for similar molecules. This applies to 

Most single bonds absorb at similar frequencies and hence the vibrations couple. The observed, pattern will depend on the carbon skeleton, and the resulting bands will originate from the oscillation of large parts of the skeleton, or the skeleton and the attached functional groups. In addition C—C stretching frequencies may also couple with C—bending vibrations (discussed in more detail in Section 4.4.4). 

The C—O stretching frequency is one of the bands that can be particularly useful for identification purposes. If no intense band appears in the fingerprint region, you can usually be sure that no C—O bonds are present. The frequency is rather variable, occurring anywhere between 1400 and 1000 cm i. 

Aromatic rings give rise to two bands at 1600 and 1500 cm They are usually sharp, but are of variable intensity, and occasionally the band at 1600 cm splits into a doublet. 

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Some experimental quantities, such as spectra, need to be transformed in some way before they are used as descriptors. For example, infrared spectra signals (IR spectra) sampled at 10 cm in the fingerprint region (1500-600 cm ) were used as vectorial descriptors, each spectrum being scaled in the range 0-100 [Benigni, Passerini et al, 1999a]. 

Infrared spectroscopy measures vibrational excitation. The energy of the incident radiation ranges from about 1 to 10 kcal mol (A 2.5 — 16.7 /xm v 600 000 cm ). Characteristic peaks are observed for certain functional groups, a consequence of stretching, bending, and other modes of vibration, and their combination. Moreover, each molecule exhibits a characteristic infrared spectral pattern in the fingerprint region below 1500 cm. ... 

The weak absorptions due to overtones in the 2000-1800 cm region are typical of a monosubstituted benzene, as are the bands in the 800-600 cm" region due to out-of-plane (OOP) bending of the C—H bonds. Ring vibrations give bands between 1600 and 1500 cm" whereas the rest of the sf>ectrum can be regarded as a "fingerprint" of the entire structure. 

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