Chapter 14: Q74P (page 527)

18-Annulene shows two signals in its 1 H NMR spectrum, one at 8.9 (12 H) and one at –1.8 (6 H) ppm. Using a similar argument to that offered for the chemical shift of benzene protons, explain why both shielded and deshielded values are observed for 18-annulene.

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Step by step solution

The protons outside the ring are strongly deshielded, whereas those inside the ring are strongly shielded.
The space around a double bond or an aromatic ring can be divided into shielding and deshielding regions, and the protons present in these regions will absorb at relatively high and low fields, respectively.

18-Annulene has 18 pi electrons that create an induced magnetic field similar to the six pi- electrons of benzene. 18-Annulene has 12 protons oriented on the outside of the ring (labeled\[{{\rm{H}}_{\rm{o}}}\]) and six protons oriented inside the ring (labeled as Hi).

The induced magnetic field reinforces the external field in the vicinity of the protons on the outside of the ring. These protons are deshielded, and so they absorb downfield (8.9 ppm).

In contrast, the induced magnetic field is opposite in direction to the applied magnetic field in the vicinity of the protons inside the ring. This shields the protons, and the absorption is very far upfield, even higher than TMS (–1.8 ppm).