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- W337284750 abstract "Among the observable phenomena, the most useful definition of aromatic nature seems to be based on the magnetic anisotropy of the system under investigation as deduced from the /sup 1/H NMR parameters. Antiaromatic 4n..pi.. systems are expected to be paratropic and to exhibit high-field shifts in the /sup 1/H NMR spectrum in contrast to the diatropic (down-field) shift of the (4n+2)..pi.. systems. Although the diatropic shift revealed by (4n+2)..pi.. electron species is well established, the experimental support for the relationship between paratropicity and antiaromaticity is by far less satisfactory. This paper presents a correlation between a theoretical and an experimental criterion for paratropicity and its relationship to the antiaromatic character of 4n..pi.. carbocyclic and heterodianions. This correlation leads to a new theoretical index for antiaromaticity, namely the extent of the HOMO-LUMO energy gap. An unequivocal correlation between the degree of paratropicity experienced by the 4n..pi.. dianions and the HOMO-LUMO energy gap is manifested by the line shape and the chemical shifts (4b) of their /sup 1/H NMR bands. The paratropicity and delocalization pattern of the dianions and their relationship to the system's topology are demonstrated. A gradual structural deviation from the symmetrical triphenylene dianion to less symmetrical systems influences themore » system's paratropicity. The partitioning of the polycyclic dianions into charged and uncharged components in a way which reduces the paratropic effect is demonstrated. The authors believe that this partitioning is another manifestation of the tendency of aromatic systems to remain so.« less" @default.
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- W337284750 date "1986-09-01" @default.
- W337284750 modified "2023-09-26" @default.
- W337284750 title "NMR studies on 4n. pi. -conjugated polycyclic anions: The relationship between the HOMO-LUMO gap, paratropicity and electronic structure" @default.
- W337284750 hasPublicationYear "1986" @default.
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