Paratropic compounds antiaromaticity

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

The doubly charged polybenzenoid molecules are model compounds for antiaromaticity. It has been pointed out that there is a direct, unequivocal correlation, between the extent of paratropicity experienced by the 4njt-conjugated systems, and the corresponding LUMO-HOMO energy gap, as estimated by SCF-MO calculations35,... 

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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