Molecules group frequency regions

The group frequency region falls approximately between 4000 to 1400 cm-1, and the absorption bands in it may be assigned to vibration of pairs of two (or sometimes three) atoms. The frequency is characteristic of the masses of the atoms involved and the nature of their bond, ignoring the rest of the molecule. Therefore, IR spectra are useful for determining the presence of functional groups in organic compounds alcohols (—OH), ketones (=CO), amines... 

An unknown compound shows vibrational absorbances occurring at 3287, 2215, and 729 cm. Keeping in mind that these are not all of the vibrational frequencies of the molecule, use the concepts of group frequency regions and correlation tables to determine whether or not such absorptions are likely for the following molecules. 

These characteristic absorption regions called group frequencies allow the analyst to detect the different elemental patterns and from them to reconstruct the molecule either by dej duct ion or by comparison with library reference spectra. The libraries contaih severaY hundred thousand spectra. 

The presence of group frequencies or "finger print" regions in infrared spectra make vibrational spectroscopy a key analytical method in identifying classes of molecules. 

Another evidence for the benefit of this structure is provided by the displacement of N02-group absorption maxima of vas and vs stretching frequencies to a lower frequency region compared to the neutral molecule (Table 3.54) [1033],... 

According to rules (1) and (2) hexamethylenetetramine C6H12N4 with the space group and one molecule per primitive cell cannot show any optically allowed lattice vibrations. Indeed, the Raman and IR spectra do not show any bands in the low-frequency region. Oxamide with one molecule per primitive cell Cj at sites C, shows three Raman bands, i.e., the three librations. Naphthalene with two molecules per primitive cell C2/, at sites C, shows six librations in the Raman spectrum. Three translational vibrations are allowed in the IR spectrum. 

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