Infrared spectra of aromatic rings

Aromatic IR spectra are messy and difficult. They show many small bands of no diagnostic value, but some are useful.

While Aromatic and Alkene C-H stretches both occur just over 3000, the C=C aromatic stretches appear between 1600 and 1450, outside the usual range for alkenes which is near 1650. Aromatic C=C stretches are often in pairs, with one at 1600 and one at 1475.

Like substituted alkenes, there are out-of-plane C-H bends between 900 and 690. These result in an overtone/combination band between 2000 and 1667, which is often used to assign how the aromatic molecule is substituted. I assume this because the overtone/combination region is less likely to be obscured by other peaks.

The shaded boxes in the table above actually correspond to out-of-plane C=C bends.

Below are some spectrum examples:

Infrared spectrum of toluene

Infrared spectrum of ortho-diethylbenzene

Infrared spectrum of meta-diethlbenzene

Infrared spectrum of para-diethylbenzene

Infrared spectrum of styrene

The in-plane bending occurs between 1000 - 1300, but these bands are not useful since they overlap with stronger bands in this region.

The original C-H out-of-plane bands can also be used to assign substitution directly in some cases, rather then using the overtone/combination region. It is suggested to use the the 900 - 690 region only for certain substituents:

Reliable substituents:
Alkyl-
Alkoxy-
Halo-
Amino-
Carbonyl-

Unreliable:
Nitro-
Carboxylic acid and derivatives
Sulfonic acids and derivatives

Monosubstituted: Always gives a strong band at 690, but in a halocarbon solvent this may be obscured by a C-X stretch. A second strong band usually appears at 750.

Orthosubstituted: One strong band near 750.

Metasubstituted: One band near 690 and one near 780. A third band is often found at 880.

Parasubstituted: One strong band between 800 and 850.