17.12 : NMR Spectroscopy of Aromatic Compounds

Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range. Consider [18]annulene, which has 12 outer hydrogens and 6 inner hydrogens. The 12 outer hydrogens are highly deshielded by the aromatic ring current and absorb downfield at a frequency of 9.3 ppm. On the other hand, the 6 inner hydrogens are highly shielded by the aromatic ring current and absorb upfield at a low frequency of −3.0 ppm.

Aromatic carbons absorb in the range of 110–150 ppm in the carbon‐13 NMR spectrum. For instance, the aromatic carbons of the benzene ring absorb at 128 ppm.