Aromatic and Heteroaromatic Rings

Publisher Summary 17. Aromatic and Heteroaromatic Rings This chapter discusses the infrared (IR) and Raman spectra of aromatic and heteroaromatic rings. A benzene ring—monosubstituted with a halogen—has a total of 30 modes of vibration. Six of these are substituent-sensitive in that the substituent atom moves appreciably or the substituent bond is significantly stretched or bent. All the rest have wave numbers within about 30 cm−1 or so of a given average wave number and can be considered to be group frequencies. Each mode has its own rules about IR or Raman intensity or wave number shift with mass or inductive or mesomeric effects of the substituents. This makes the substituted benzenes some of the most complex groups encountered. The chapter illustrates the major substituent-insensitive mono-, di-, and tri-substituted benzene IR and Raman bands. The out-of-plane C—H wag vibrations give rise to the most useful IR bands and are used to distinguish different types of aromatic ring substitution.