Double bond magnetic anisotropic effect

Figure 1.13. The magnetic anisotropic effect of the carbon-carbon double bond, the carbonyl group, and the nitro group, and the resulting shielding (+) and deshielding (—) regions.

The electrons modify the magnetic field experienced by the nucleus. Chemical shift is caused by simultaneous interactions of a nucleus with surrounding electrons and of the electrons with the static magnetic field B0. The latter induces, via electronic polarization and circulation, a secondary local magnetic field which opposes B0 and therefore shields the nucleus under observation. Considering the nature of distribution of electrons in molecules, particularly in double bonds, it is apparent that this shielding will be spatially anisotropic. This effect is known as chemical shift anisotropy. The chemical shift interaction is described by the Hamiltonian... 

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