[1] This is a direct consequence of Ampère's law; since the electrons involved are free to circulate, rather than being localized in bonds as they would be in most non-aromatic molecules, they respond much more strongly to the magnetic field.
[2] The effect helps distinguish these nuclear environments and is therefore of great use in molecular structure determination.
In contrast any proton inside the aromatic ring experiences shielding because both fields are in opposite direction.
A similar effect is observed in three-dimensional fullerenes; in this case it is called a sphere current.
The nucleus-independent chemical shift (NICS) is a computational method that calculates the absolute magnetic shielding at the center of a ring.
The values are reported with a reversed sign to make them compatible with the chemical shift conventions of NMR spectroscopy.
A diagram of an aromatic ring current.
B
0
is the applied magnetic field, the red arrow indicating its direction. The orange ring shows the direction of the ring current, and the purple rings show the direction of the
induced
magnetic field.
Magnetically induced probability current density vectors in benzene (C
6
H
6
) calculated explicitly using
quantum chemical methods
.
B
0
is set perpendicular to the molecular plane, in the left subfigure only vectors in the molecular plane are shown, in the right subfigure only vectors 1 a.u. (~52 pm) above the molecular plane are shown. Only vectors with a
modulus
between 0.01 and 0.1 nA/T are displayed. Contrasting the schematic picture, which gives in analogy to the
laws of classical electrodynamics
only diatropic contributions, the full quantum mechanical picture also yield paratropic contributions, as counter-clockwise vortices in this diagram. These are located in benzene mainly in the molecular plane, inside the C
6
ring.
