Aromatic compounds diamagnetic current

This ring current in benzene is termed diamagnetic and is characteristic of aromatic compounds in general. The presence of a diamagnetic ring current provides a useful experimental criterion for the presence of aromaticity in a compound. Other examples of the use of this method are provided in Section 16.10. 

Aromatic compounds display magnetic properties related to a ring current. These include unusual chemical shifts, especially of protons, large magnetic anisotropies, and large diamagnetic susceptibility exaltations. 

Aromatic compounds are defined as cyclic or polycyclic systems which sustain a diamagnetic ring current and consequently exhibit a total diatropic lowfield 1H NMR chemical shift relative to that of vinylic protons. Paramagnetic ring current is expected and was shown to be induced in antiaromatic species and to result in a total highfield H NMR band displacement, while non aromatic compounds give rise to characteristic vinylic XH patterns. 

We can define an aromatic compound - - as a compound which will sustain an induced (diamagnetic) ring current" [35]. Evidence for ir-electron delocalisation in cyclic polyenes is also forthcoming from measures of coupling constants, especially the differences between the coupling constants across neighbouring bonds. If these are markedly different... 

The aromatic compounds generate a diamagnetic (diatropic) current opposites to the applied magnetic field (Bq) whereas the antiaromatic compounds generate a paramagnetic current (paratropic), which enforces the effect of the applied magnetic field [48,50-54], The diatropic and paratropic terms refer to their effects in the chemical shift of a trial nncleus [33],... 

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