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Diamagnetic anisotropy is an effect caused by the pi electrons inducing their own magnetic field when placed under an external magnetic field.

The pi electrons aren't held as tightly together and so behave differently to sigma electrons.

A diagram showing the anisotropic induced magnetic field for a double bond as well as an aromatic system. The examples used in the diagram are a carbon to carbon double bond and a benzene ring. The is an arrow pointing north labeled B subscript 0 representing the applied magnetic field. On both the double bond and the aromatic ring there is a circle above and below with an arrow point east to represent the circulating electrons. The anisotropic induced field is represented by two ovals side by side, with a series of small red arrows showing the direction. Inside the center of the double bond and the aromatic ring, the direction of the anisotropic field opposes the applied field whereas on the outer region it aligns with the applied field.

Figure 1:图显示出抗磁性各向异性双bonds and aromatic systems

One region of the induced field opposes the applied field which is equivalent to the effect of shielding. The other aligns with the direction of the field which has an equivalent effect to deshielding.

This is why in carbon to carbon double bonds we will see varying chemical shifts for both carbons.

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