π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

In aromatic compounds, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around their perimeters.  

This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside.

So, the protons in benzene are deshielded and exhibit high chemical shifts from 6.5–8.5 ppm.

The shielding effect at the center of aromatic rings is evident in complex molecules, such as annulenes.

In [18]annulene, protons outside the ring are deshielded and resonate at δ 9.3, while those at the center resonate around δ −3.0, below TMS.

In anti-aromatic compounds with 4n π-electrons, the ring current is paratropic and flows in the opposite direction to that observed in aromatic compounds.

The consequent change in the induced field direction shields protons outside the ring and deshields those inside, as seen in [16]annulene.