Fingerprint region, infrared absorption

Fig. 5. A summary of the infrared absorption bands exhibited by hydrocarbon ligands on metal atoms in various model compounds. Surface species on metals may give absorptions varying by ca. 50 cm 1 from the band positions in the model-compound spectra in the fingerprint region below 1400 cm. The patterns of band-positions and intensities are significant. M indicates MSSR-allowed modes for an analogous species on a flat surface when the adsorbed species is on a site of high symmetry (--) indicates other absorptions that may occur for adsorption on less symmetrical sites or on small metal particles, vs—very strong s—strong ms—medium strong m—medium mw—medium weak w—weak.

FIGURE 2.6 Infrared absorption spectrum obtained in the fingerprint region for clopidogrel bisulfate, Form-I. 

Fingerprint region (Section 13.9) The region below 1500 cm-1 in the infrared spectrum that contains numerous absorptions due to single bond stretches and a variety of bending vibrations. Because of the large number of bands, this is the region where comparison of the spectrum of an unknown to that of a known compound can establish the identity of the unknown. 

The region 1400-650 cm-1 is known as the fingerprint region and is usually checked for identification as absorptions in this region are characteristic of a substance. This can be compared to infrared spectra of known substances for identification. 

Infrared spectra of solutions are extensively used for identification and characterization of organic compounds. The features between 200 and 1400 cm , known as the fingerprint region, are especially useful. These are absorptions due mainly to bending modes. Although it is difficult to assign the individual bonds producing this spectrum, each compound produces a characteristic pattern which can serve to identify the molecule. Fig. 14.5 shows the IR spectra of 1-propanol... 

Two substances that have identical infrared spectra are, in elTect, identical in thousands of different physical properties the absorption oflight at thousands of different frequencies—and must almost certainly be the same compound. (One region of the infrared spectrum is called, appropriately, the fingerprint region.)... 

The fingerprint region contains numerous and frequently overlapping absorption bands, the exact assignments of which are normally impossible even when a compound of known structure is analysed. The term fingerprint is employed because the pattern of absorption in that part of the spectrum is uniquely characteristic of the compound concerned. The main value of this region is in establishing the identity of samples obtained from different sources. If two samples are identical then their infrared spectra will be exactly superimposable when measured under the same conditions. 

A rapid FTIR method for the direct determination of the casein/whey ratio in milk has also been developed [26]. This method is unique because it does not require any physical separation of the casein and whey fractions, but rather makes use of the information contained in the whole spectrum to differentiate between these proteins. Proteins exhibit three characteristic absorption bands in the mid-infrared spectrum, designated as the amide I (1695-1600 cm-i), amide II (1560-1520 cm-i) and amide III (1300-1230 cm >) bands, and the positions of these bands are sensitive to protein secondary structure. From a structural viewpoint, caseins and whey proteins differ substantially, as the whey proteins are globular proteins whereas the caseins have little secondary structure. These structural differences make it possible to differentiate these proteins by FTIR spectroscopy. In addition to their different conformations, other differences between caseins and whey proteins, such as their differences in amino acid compositions and the presence of phosphate ester linkages in caseins but not whey proteins, are also reflected in their FTIR spectra. These spectroscopic differences are illustrated in Figure 15, which shows the so-called fingerprint region in the FTIR spectra of sodium caseinate and whey protein concentrate. Thus, FTIR spectroscopy can provide a means for quantitative determination of casein and whey proteins in the presence of each other. 

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. 

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