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Aromaticity of Rees-type Hydrocarbons - a DFT Computational Study

The structure and aromatic properties of Rees hydrocarbons 7bH-cyclopent[cd]indene and its benzo-annelated derivative 1a and 2a, respectively, are examined by the B3LYP/6-31+G(d) calculations employing HOMA criterion of Krygowski and coworkers. It is shown that 1a possesses strong pi-electron delocalization over the perimeter of the CC bonds, thus forming a quasi-[10]annulene pattern. Its aromatic character is determined to be 83%. In contrast, 2a is less convenient model system for [14]annulene. The reason behind is that the perimeter network of the potentially aromatic 14 pi-electrons is supplemented by two additional more local aromatic patterns involving 10pi and 6pi electrons. Consequently, the pi-electron delocalization over the molecular rim is incomplete being thus diminished. The aromatic character of the peripheral bonds in both 1a- and 2a- anions formed upon deprotonation of the central C-H bond is decreased, since the role of the smaller rings in forming aromatic subsystems is increasing. Finally, polycyano substitution of 1a and 2a decreases aromaticity due to a price to be paid for the resonance effect taking place between the carbocyclic pi-network and the double bonds of the CN groups. The resonance effect is particularly strong in anions derived by heterolytic cleavage of the C-H bond emanating from the central sp3 carbon atom.

Rees hydrocarbons; [10]annulene; [14]annulene; aromaticity; anions; resonance effect

Thematic issue: Electron delocalization and aromaticity