Group frequencies, infrared spectrum interpretation

The following summary provides a recommended approach to the interpretation of an unknown spectrum which may be adopted until experience has developed an intuitive appreciation of the characteristics of infrared spectra. It should be used in association with the more detailed notes which follow, describing the way in which characteristic group frequencies arise and the variations in frequency position which accompany environmental changes. 

Once an infrared spectrum has been recorded, the next stage of this experimental technique is interpretation. Fortunately, spectrum interpretation is simplified by the fact that the bands that appear can usually be assigned to particular parts of a molecule, producing what are known as group frequencies. The characteristic group frequencies observed in the mid-infrared region are discussed in this chapter. The types of molecular motions responsible for infrared bands in the near-infrared and far-infrared regions are also introduced. 

Infared Analysis. The molecule is 4-chlorobenzaldehyde.The infrared spectrum of this aromatic aldehyde (Fig. 6.18, page 182) is rich and interesting.The aromatic aldehyde macro group frequency train (see Strategies for Interpreting Infrared Spectra) consists of peaks near 3090, 3070, 2830 and 2750, 1706, 1602, 1589, and 1396 cm ... 

In Sections 13.2-13.26, an orderly procedure is outlined for the less experienced for the initial broad interpretation of an unknown infrared spectrum. This initial survey will eliminate some categories and should indicate some categories of group frequencies that need a more detailed study. 

Eischens and Pliskin have interpreted the infrared spectra of ethylene chemisorbed on nickel dispersed on silica 32). When introduced to a surface previously exposed to hydrogen, ethylene gave rise to absorption bands which correspond to the C—H stretching frequencies of a saturated hydrocarbon (3.4-3.5 p) and a deformation associated with a methylene group (6.9 p). A weak band at 3.3 p was attributed to an ole-finic C—H. Treatment of the chemisorbed ethylene with hydrogen caused the spectrum to change to one which was interpreted as due to an adsorbed ethyl radical. Apparently in the presence of hydrogen most of... 

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