NMR STUDIES ON 4n pi -CONJUGATED POLYCYCLIC ANIONS: THE RELATIONSHIP BETWEEN THE HOMO-LUMO GAP, PARATROPICITY AND ELECTRONIC STRUCTURE.

The 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 chemical shifts of their **1H 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 the system's paratropicity. The partitioning of the polycyclic dianions into charged and uncharged components in a way which reduces the paratropic effect are demonstrated.