Local Aromaticity (local + aromaticity)

Distribution by Scientific Domains

Selected Abstracts

Chemical graph theory and n -center electron delocalization indices: A study on polycyclic aromatic hydrocarbons

Marcos Mandado
Abstract Relations between aromaticity indices derived from chemical graph theory and those based on 6-center electron delocalization are investigated for a series of polybenzenoid hydrocarbons. Aromatic stabilization obtained by means of the effective scaled electron delocalization is highly correlated to the resonance energy, RE, obtained both from SCF MO calculations and conjugated ring circuits model. Local aromaticity of benzene rings is discussed using two different criteria, in one of them aromaticity is just given by the cyclic , -electron conjugation of the ring, whereas terms involving more than one ring are also considered in the other one. Indices derived from chemical graph theory and those obtained from the 6-center electron delocalization give rise to the same local aromaticity. Moreover, 6-center electron delocalization provides more quantitative information. 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source]

Substituent effect on local aromaticity in mono and di-substituted heterocyclic analogs of naphthalene

Afshan Mohajeri
Abstract A quantitative study on local aromaticity has been performed on a series of mono- and di-substituted biheterocycles (quinoline, isoquinoline, quinoxaline, quinazoline). Three electronically based indices (PDI, ATI, and FLU) have been employed to investigate the substituent effect on the , -electron delocalization in both heterocycle and benzenoid rings. Three typical substituents (Cl, OCH3, and CN) with different inductive and resonance power have been selected. Generally, substituent causes a reduction in aromaticity irrespective of whether it is electron attracting or electron donating. It is shown that the maximum aromaticity exhibits a similar trend of Cl,>,CN,>,OCH3 for all the studied rings. Moreover, it is found that the substituent situation with respect to the heteroatom has a significant influence on the aromaticity. It results from our study that in di-substituted derivatives, irrespective of whether the two substituents form a meta or para isomer, they preferably choose the position which leads to the maximum aromaticity character. Copyright 2009 John Wiley & Sons, Ltd. [source]

Aromatic stabilization in heterofullerenes C48X12 (X,=,N, P, B, Si),

Zhongfang Chen
Abstract B3LYP density functional calculations were performed on two S6 symmetrical isomers (I and II) of C48X12 (X=N, B, P, Si) heterofullerenes, and their global and local aromaticity were evaluated by nucleus-independent chemical shifts (NICS). Despite the unfavorable heteroatom repulsive interactions, isomer II is more stable than I owing to the combination of global and local aromaticity. The latter arises from the presence of triphenylene units in isomer II. The aromatic stabilization effects found in this study should be taken into account when predicting the most stable isomers of heterofullerenes. The same is true for predictions of the isomers of fullerene adducts such as C60Cl12. Copyright 2003 John Wiley & Sons, Ltd. [source]

The Breakdown of the Minimum Polarizability Principle in Vibrational Motions as an Indicator of the Most Aromatic Center

Miquel Torrent-Sucarrat Dr.
Abstract The vibrational motions that disobey the minimum polarizability principle (MPP) in ,-conjugated molecules are distortions of the equilibrium geometry that produce a reduction in the polarizability due to the localization of , electrons. For aromatic species, this electronic localization is responsible for the subsequent reduction in the aromaticity of the system. In the present work, we diagonalize the Hessian matrix of the polarizability with respect to the vibrational nontotally symmetric normal coordinates, to calculate the nontotally symmetric distortions that produce the maximum breakdown of the MPP in a series of twenty polycyclic aromatic hydrocarbons. It is shown that the nuclear displacements that break the MPP have larger components in those rings that possess the highest local aromaticity. Thus, these vibrational motions can be used as an indicator of local aromaticity. [source]