Aromatic CH Stretching Frequencies

CH stretching frequencies associated with the benzene ring or other aromatic rings appear near 3030 cm Since this is in the region of the olefinic CH stretch, the aromatic CH vibration can be confused with the olefinic CH unless other aromatic vibrations are identified. The aromatic CH stretch is usually a band of medium to weak intensity. For some aromatic compounds several bands can be found near 3030 cm When a sodium chloride monochromator is used, the aromatic CH stretch may not be resolved from the stronger alkane CH vibrations. 

Substitution of elements and groups such as O, F, and NO2 in the ring breaks down these patterns to some extent, since such electronegative groups can shift these bands as well as introduce new absorption bands. Moreover, if the compound contains a carbonyl group, the bands may be obscured, since the carbonyl frequency is very strong in this region. 

For specific examples of cases that either do or do not permit these vibrations to be identified, let us re-examine the data presented in Table 5-XIII. It should be recalled that the vibrations marked with asterisks are those which are difficult to assign due to interference by other vibrations. 

For some of the compounds listed, we find that either the 1600 or the 1585 cm band is sufficiently strong to be identified, but that the other is either weak or absent. In general, at least one strong band is found in the 1620-1570 cm region. In compounds with NH, NH2, or NO2, and in some cases C=C or C=0 groups, the 1600 cm band can be obscured by vibrations from these groups. Several examples of this interference are listed in Table 5-XIII. 

Many nonbenzenoid compounds can have strong bands in the 1600-1450 cm region. For example, the alkene CH2=CHCH(CH3)CH2CH3 

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