Diamagnetic circulation:

A unique type of secondary magnetic effect is termed as diamagnetic circulation. This takes place in functional groups that consist of π bonds. As an instnace, we shall consider the aromatic ring.

Figure: Diamagnetic circulation of an aromatic ring.

The applied external magnetic field (Bo) employed in nmr has an interesting influence on the π-electrons of the aromatic ring, causing them to circulate round the ring in a process termed as diamagnetic circulation. In turn this movement sets up a secondary magnetic field (Be) presented through the force lines displayed figure. The direction of these lines opposes the applied magnetic field at the center of the aromatic ring, but improves it at the edges where the aromatic protons are situated. This means that the field examined by the aromatic protons is increased (Bo+Be) causing their precessional frequency to increase. Here this means that a greater energy is needed for resonance, resultant in a higher chemical shift.