π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as annulenes. In [18]annulene, protons outside the ring are deshielded and resonate at δ 9.3, while those at the center are so strongly shielded that they resonate at δ −3.0, below TMS. In anti-aromatic compounds with 4n π-electrons, the ring current flows in the opposite direction to that observed in aromatic compounds and is called the paratropic ring current. The induced field is also in the opposite direction. Thus, protons outside the ring are shielded in compounds like [16]annulene while those inside the ring are deshielded.