Group vibration characteristic absorption band

What information can we derive about molecular structure from the vibrational bands of infrared spectra Absorption of radiation in the range of 5000-1250 cm-1 is characteristic of the types of bonds present in the molecule, and corresponds for the most part to stretching vibrations. For example, we know that the C—H bonds of alkanes and alkyl groups have characteristic absorption bands around 2900 cm-1 an unidentified compound that shows absorption in this region will very likely have alkane-type C—H bonds. 

Since IR spectra are essentially due to vibrational transitions, many substituents with single bonds or isolated double bonds give rise to characteristic absorption bands within a limited frequency range in contrast, the absorption due to conjugated multiple bonds is usually not characteristic and cannot be ascribed to any particular grouping. Thus IR spectra afford reference data for identification of pyrimidines, for the identification of certain attached groups and as an aid in studying qualitatively the tautomerism (if any) of pyrimidinones, pyrimidinethiones and pyrimidinamines in the solid state or in non-protic solvents (see Section 2.13.1.8). 

The interactions of photons with molecules are described by molecular cross-sections. For IR spectroscopy the cross-section is some two orders of magnitude smaller with respect to UV or fluorescence spectroscopy but about 10 orders of magnitude bigger than for Raman scattering. The peaks in IR spectra represent the excitation of vibrational modes of the molecules in the sample and thus are associated with the various chemical bonds and functional groups present in the molecules. The frequencies of the characteristic absorption bands lie within a relatively narrow range, almost independent of the composition of the rest of the molecule. The relative constancy of these group frequencies allows determination of the characteristic... 

For pure Si-MCM-41. this band has been assigned to the Si-O stretching vibrations and the presence of this band in the pure siliceous is due to the great amount of silanol groups present. A characteristic absorption band at about 970 cm 1 has been observed in all the framework IR spectra of titanium-silicalites. It was also reported that the intensity of 970 cm 1 band increased as a function of titanium in the lattice[17] and this absorption band is attributed to an asymmetric stretching mode of tetrahetral Si-O-Ti linkages [18] in the zeolitic framework. The increase in intensity of this peak with the Ti content has been taken as a proof of incorporation of titanium into the framework. 

The characteristic absorption band, which corresponds to the OH valence vibration for the free group, is situated at about 2.7 [x. This is observed in the gaseous state and in very dilute solutions in non-polar solvents, such as CS2, hexane etc. for water, alcohols and organic acids. 

The vibrational spectra clearly indicate the presence of cyanamide groups, with characteristic absorptions at 2095 - 2120 cm [17]. Also, the bands for C-H and Si-CH3 groups at 2890 -2960 cm and 1250 cm are detected. 

Fig. 2 relates the IR absorption spectra of dextran -MMA copolymer, the starting material dextran and side chain PMMA. The spectrum of the copolymer has some characteristic absorption band (a), (b) and (c) the band at around 3410 cm attributed to 0-H stretching vibration of dextran, the band at 1725 cm attributed to the carbonyl -group of PMMA, and the band at 1000 to 1150 cm" attributed to the pyranose ring of dextran. The spectrum of dextran alone has no absorption band at 1720 cm due to the stretching vibration of C=0, and the spectrum of PMMA alone has no absorption band at 3410 cm . ... 

The situation is different for the deposit trinuclear Ru hexaacetate complex without M-M bonds. The IR spectrum of this complex contains no characteristic absorption bands of carboxyl groups. In the range of the carrier s stretching vibrations, we observe the same changes as described above for immobilized binuclear Ru tetraacetate. Thus, in contrast to binuclear complexes with the M-M bond, we may suppose that heterogenization of the trinuclear acetate complex without Ru-Ru bonds is accompanied by decomposition the initial complex turns into a mononuclear one, and the acetate groups are substituted by the carrier s amino groups. 

In a quantum mechanical description, the simple spring-like picture of chemical bonds, of course, breaks down and the molecule has to be described as a many-body system of interacting particles including electrons and nuclei. Nevertheless, the normal mode vibrations have their counterpart in the fundamental excitations of the nuclear vibrational degrees of freedom (DOF) of the molecule. The fundamentals can be excited by infrared radiation (IR) and characteristic absorption bands in the IR spectra immediately point to the existence of certain chemical bonds or to functional groups and hence IR (and Raman) spectroscopy are powerful tools to investigate and study the chemical structure of molecules. 

IR spectra can be quite complex because the stretching and bending vibrations of each bond in a molecule can produce an absorption band. Organic chemists, however, do not try to identify all the absorption bands in an IR specttum. They tend to focus on the functional groups. In this chapter, we will look at several characteristic absorption bands so you will be able to tell something about the structure of a compound that gives a particular IR spectrum. 

Some typical results are shown in Table 1. Polymers are soluble in organic solvents such as benzene, toluene, THF and chlorinated hydrocarbons. The IR spectrum of these polymers displays characteristic absorption bands at 1640 cm (C=C stretching), 1600 and 1500 cm"l (phenyl ring vibrations), 1235 cm" (phenyl ether stretching) and 1015 cm (aliphatic ether stretching). The NMR spectrum of these polyethers recorded in CDCl at room temperature exhibits multiplets between 7.20 and 6.67 ppm (aromatic protons) and peaks at 6.02 ppm (-CH=CH-), at 4.50 ppm (-CH -0) and 1.60 ppm (-CH ). A small peak is observed at 4.05 ppm which can be attributed to the protons of the chloromethyl end groups. This peak is absent in the spectrum of sample 6 (Table 1). 

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