Aromaticity of Rees-type hydrocarbons—a DFT computational study

Abstract

The structure and aromatic properties of Rees hydrocarbons 7bH-cyclopent [cd]indene and its benzoannelated 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 p-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 14p-electrons is supplemented by two additional more local aromatic patterns involving 10p and 6p electrons. Consequently, the p-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 p-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 sp 3 carbon atom.

Polycyano Derivatives of some Organic Tri- and Hexacyclic Molecules Are Powerful Super- and Hyperacids in the Gas Phase and DMSO: Computational Study by DFT Approach

“…Recently, we have demonstrated that Rees compound I is a quasi-[10]annulene, since it possesses strong π-electron delocalization over the CC perimeter. Its aromatic character was determined to be 83% employing the geometric HOMA (harmonic oscillator model of aromaticity) criterion, proposed by Krygowski and co-workers .…”

Geometric Aspects of Aromaticity: Interrelations between Intramolecular Hydrogen Bonds, Steric Effects and π‐Electron Delocalisation in Nitroanilines

“…[23] For the nonplanar molecule 7bH-cyclopent-[cd]indene, a HOMA value of 0.83 was found. [16] The lower HOMA values correspond to compounds with a nonplanar benzene ring. A trend of increasing ΔP values with decreasing HOMA is observed ( Figure 2); however, a decrease in aromaticity is not always paralleled by a distortion of the benzene plane.…”

“…The HOMA parameters were applied to the study of aromaticity of nonplanar molecules. [16,17] The relationship between these factors in sterically crowded aromatic compounds with NH···O intramolecular hydrogen bonds is the object of this study. As model compounds, we chose substituted 2,4,6-trinitroanilines (Scheme 1).…”

Interplay of thermochemistry and Structural Chemistry, the journal (volume 18, 2007) and the discipline

“…Vianello and Maksić [389] studied the structure and aromatic properties of the Rees-type hydrocarbons, 7bH-cyclopent[cd]indene and its benzoannelated derivatives. Such species include ''fluoradene'' [390,391] for which no enthalpy of formation has been reported in the experimental literature, a surprising omission given its exceptional acidity [392,393].…”