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| - Role of Electron Density and Magnetic Couplingson the Nucleus-Independent Chemical Shift(NICS) Profiles of [2.2]Paracyclophane andRelated Species
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| - The nucleus-independent chemical shifts (NICS) and electrondensity profiles along the inner and outer regions definedby the two stacked aromatic rings of [2.2]paracyclophanehave been analyzed and compared to those of free benzeneand p-xylene and benzene dimers taken as reference models.It is found that stacked aromatic rings show a reduction ofthe NICS indicator of aromaticity as compared to the samefree aromatic systems. This decrease of the NICS values uponstacking is not due to an increase of the electron density inthe inner region between the rings (as claimed in a previouswork) but is related to the magnetic couplings betweensuperimposed rings that affect this measure of local aromaticity. The increase of local aromaticity in superimposedaromatic rings indicated by NICS is not real but the resultof the coupling between the magnetic fields generated bythe two stacked rings. This result warns about the use ofNICS as a descriptor of aromaticity for species havingsuperimposed aromatic rings.
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