Abraham Minsky

Paratropicity and antiaromaticity: role of the homo-lumo energy gap

Abstract Doubly charged systems derived from fused benzenoid polycycles reveal an unquenched delocalization of 4 n π-electrons and hence are predicted to possess antiaromatic character. The magnitude of the paratropic 1 H NMR chemical shifts, due solely to the paramagnetic secondary field sustained in these species, was found to depend linearly upon the magnitude of LUMO-HOMO energy gaps of the corresponding systems. The existence of such a correlation enables a comprehensive treatment of the various factors which determine the antiaromatic character and the subtle interrelations between those factors. This, in turn, leads to a deeper understanding of antiaromaticity.

1H NMR patterns of antiaromatic systems: Paratropic displacement dependence upon LUMO-HOMO energy gap

Abstract 1 H NMR chemical shifts of antiaromatic species reveal an enhanced paratropic displacement. The high-field shifts exhibited by doubly charged benzenoid polycycles were shown to be strongly related to the LUMO-HOMO energy gaps in these antiaromatic systems. As the gap decreases, a larger paratropic shift was observed.

Polycyclic aromatic ions

The study of charged polycyclic systems enables a better understanding of the nature of the Tr—delocalization in these series. Pyrene (1) being aromatic and having l6v—electrons demonstrates the two approaches to its aromaticity i.e. the contribution of the components and the peripheral conjugation of l4Tr—electrons. The preparation and study of anionic species homoelectronic and isoelectronic to pyrene is reported. The formation of pyrene dianion and the surprising formation of the diatropic pyrene tetra— anion demonstrate the importance of the peripheral model. It is shown that some tetra—anions are asymmetric probably due to the influence of the counter ion. This phenomenon is related to the charge densities and their residence on the molecule.

Diindeno[cd:1m]perylene dianion. Local vs. peripheral contributions to the aromatic character

Diindeno[cd;lm]perylene (1) is shown to undergo a two electron reduction process to its dianion 2. Both neutral and doubly charged systems exhibit an enhanced diamagnetic character which is believed to be acquired via two different mechanisms.

Protonation of acenaphthoquinone. A novel peripheral 10 π-electron aromatic system

Acenaphthoquinone (1) undergoes ppotonation to afford the respective double protonation product 2. The proton and carbon NMR parameters of 2 are discussed.