Aromatic rings substitution patterns, infrared

The clustering seems to Indicate that ring substitution patterns are recognized almost as easily as the substituents themselves That the clustering calculations display this Information correlates with the determination of aromatic ring substitution patterns from Infrared spectra practiced by chemists.(25) The results also Indicate that It may be best to consider the compounds with chlorines on alkyl side chains as a separate class. 

Benzaldehyde also possesses a second powerful macro group frequency [www]-> train that reflects the substitution pattern of the aromatic ring system (see Infrared Discussions). The monosubstituted benzene ring macro group frequency train contains peaks in the following regions 1950,1880,1800, 1730, 750, and 690 cm. ... 

Some chemical structures exhibit typical distances that occur independently of secondary features, which mainly affect the intensity distribution. In particular, aromatic systems contain at least a distance pattern of ortho-, meta-, and para-carbon atoms in the aromatic ring. A monocyclic aromatic system shows additionally a typical frequency distribution. Consequently, Cartesian RDF descriptors for benzene, toluene, and xylene isomers show a typical pattern for the three C-C distances of ortho-, meta-, and para-position (1.4, 2.4, and 2.8 A, respectively) within a benzene ring. This pattern is unique and indicates a benzene ring. Additional patterns occur for the substituted derivatives (3.8 and 4.3 A) that are also typical for phenyl systems. The increasing distance of the methyl groups in meta- and para-Xylene is indicated by a peak shift at 5.1 and 5.8 A, respectively. These types of patterns are primarily used in rule bases for the modeling of structures explained in detail in the application for structure prediction with infrared spectra. 

Infrared Analysis. The infrared spectrum of 5-nitrosalicylic acid (Fig. 6.26 on page 238) is characteristic of aromatic carboxylic adds (see discussion in Experiment [7]). Note that (1) the substitution of the ring is revealed by the presence of the 1,2,4-combination band pattern with peaks at 1940,1860, and 1815 cm (2) the strongest band in the spectrum below 1750 cm is assigned to the symmetric stretch of the —NO2 group found at 1339 cm , and (3) the conjugated carboxyl C=0 stretch is located at 1675 cm . ... 

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