Aromaticity of Rees-type hydrocarbons—a DFT computational study

Autor: Robert Vianello, Zvonimir B. Maksić
Rok vydání: 2007
Předmět:
Zdroj: Structural Chemistry. 18:821-826
ISSN: 1572-9001
1040-0400
DOI: 10.1007/s11224-007-9233-3
Popis: 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 π-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 14π-electrons is supplemented by two additional more local aromatic patterns involving 10π and 6π electrons. Consequently, the π-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 the price paid for the resonance effect taking place between the carbocyclic π-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.
Databáze: OpenAIRE
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