17.12 : 芳香族化合物的核磁共振波谱分析

¹H and ¹³C NMR spectroscopy are very helpful in identifying aromatic compounds.

Typically, aromatic hydrogens are highly deshielded by the aromatic ring current.

As a result, they exhibit characteristic ¹H NMR absorptions in the range of 6.5–8 ppm.

For instance, aromatic hydrogens of benzene absorb at 7.3 ppm.

However, aromatic hydrogens of large rings absorb farther upfield or downfield.

Consider [18]annulene. It has 12 hydrogens outside the ring, which are highly deshielded by the aromatic ring current. The 6 hydrogens inside the ring are highly shielded.

Consequently, the outside hydrogens absorb downfield at a high frequency. The inside hydrogens absorb upfield at a low frequency.

Aromatic carbons exhibit ¹³C NMR absorptions in the range of 110–150 ppm.

For instance, aromatic carbons of benzene absorb at 128 ppm.