Infrared spectroscopy characteristic absorption region

Infrared spectroscopy (IR) is also a very useful technique for characterizing stmcture. IR spectroscopy can be used for identification purposes as well as for monitoring the progress of a chemical reaction. Comparisons of the positions of absorptions in the IR spectrum of a sample with the characteristic absorption regions, leads to identification of the bonds and functional groups present in the sample. For example, the chemical stmctures of polyimide-silica hybrid films were confirmed by IR spectroscopy by the appearance of two absorptions at 1,100 and 830 cm indicating the formation of the Si-O bonds. 

Transmission Fourier Transform Infrared Spectroscopy. The most straightforward method for the acquisition of in spectra of surface layers is standard transmission spectroscopy (35,36). This approach can only be used for samples which are partially in transparent or which can be diluted with an in transparent medium such as KBr and pressed into a transmissive pellet. The extent to which the in spectral region (typically ca 600 4000 cm ) is available for study depends on the in absorption characteristics of the soHd support material. Transmission ftir spectroscopy is most often used to study surface species on metal oxides. These soHds leave reasonably large spectral windows within which the spectral behavior of the surface species can be viewed. 

The structure of the protonated enamines has been investigated by infrared spectroscopy. On protonation there is a characteristic shift of the band in the double-bond stretching region to higher frequencies by 20 to 50 cm with an increased intensity of absorption (6,13,14a). Protonated enamines also show absorption in the ultraviolet at 220-225 m/x due to the iminium structure (14b). This confirms structure 5 for these protonated enamines, because a compound having structure 4 would be expected to have only end absorption as the electrons on nitrogen would not be available for interaction with the n electrons of the double bond. 

The wavelengths of IR absorption bands are characteristic of specific types of chemical bonds. In the past infrared had little application in protein analysis due to instrumentation and interpretation limitations. The development of Fourier transform infrared spectroscopy (FUR) makes it possible to characterize proteins using IR techniques (Surewicz et al. 1993). Several IR absorption regions are important for protein analysis. The amide I groups in proteins have a vibration absorption frequency of 1630-1670 cm. Secondary structures of proteins such as alpha(a)-helix and beta(P)-sheet have amide absorptions of 1645-1660 cm-1 and 1665-1680 cm, respectively. Random coil has absorptions in the range of 1660-1670 cm These characterization criteria come from studies of model polypeptides with known secondary structures. Thus, FTIR is useful in conformational analysis of peptides and proteins (Arrondo et al. 1993). 

In practice, UV absorption spectroscopy (in the region from 200 to 380 nm) is for the most part yields information only about the conjugated system present in the molecule. However, when taken in conjunction with the wealth of detail provided by infrared (IR) and nuclear magnetic resonance (NMR) bands may lead to successful structural elucidations. The principal characteristics of an UV absorption band are its position (2max) and intensity (emax or log emax). 

Infrared spectroscopy is the favored technique for characterizing borates, and several compilations of data have been made (171, 333,417, 423,431,432). The intense absorption at about 970 cm-1 is characteristic of tetrahedrally coordinated boron, as measured for the magnesium borates MgO B203 nH20 (153), and bands at 700 to 900 and 1200 to 1500 cm-1 can correspond to triangular boron coordination. Absorptions in the region 400 to 700 cm-1 have been attributed to chain structures (423). 

The Infrared Region 515 12-4 Molecular Vibrations 516 12-5 IR-Active and IR-lnactive Vibrations 518 12-6 Measurement of the IR Spectrum 519 12-7 Infrared Spectroscopy of Hydrocarbons 522 12-8 Characteristic Absorptions of Alcohols and Amines 527 12-9 Characteristic Absorptions of Carbonyl Compounds 528 12-10 Characteristic Absorptions of C—N Bonds 533 12-11 Simplified Summary of IR Stretching Frequencies 535 12-12 Reading and Interpreting IR Spectra (Solved Problems) 537 12-13 Introduction to Mass Spectrometry 541 12-14 Determination of the Molecular Formula by Mass Spectrometry 545... 

Results and Discussion. The application of infrared spectroscopy to the analysis of SAN copolymers in solution has two limitations the poor solubility of the polymers, which constrains the analysis to those regions where the solvents are transparent and the poor sensitivity of the infrared at low concentrations. This last problem is more severe when the characteristic absorption bands have medium or weak intensities (typically -C = N stretching). Of particular interest are the bands between 3-7 jim, some of which can be detected in a variety of solvents and provide information on the AN/St ratio. The bands at 13 and 14 fim are very strong and amenable to detection particularly in SEC applications. These bands contain information on the styrene concentration and the molecular structure (Hummel 1974). 

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. 

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