Local aromaticity

A quantitative study on local aromaticity based on n-center electron delocalization indices, wheren is the number of atoms in the ring, was performed on benzo[c]furan and benzo[b]furan. 

Some indices aim to measure the local aromaticity of an individual ring in a polycyclic system, while others measure the global aromaticity of the whole molecule. 

One can also localize aromatic systems by applying the localization procedure to 7r electrons and a electrons simultaneously. The sigma electrons will then be localized in the regions of the single bonds. Since the localization energy to be gained from the n electron localizations is less than that from the sigma electron localization, the total localization will be dominated by the latter. 

It was Clar [5] who invented a novel usage of the circles and arrows on the Kekule structure of aromatic hydrocarbons to represent the stability of the molecule and local aromatic characters based on his own numerous experimental results and empirical rules [6], but not on sophisticated quantum-chemical treatment. According to his scheme the ground state of I and II can well be represented by the following Clar formulas, Ic and lie,... 

Randic [21] and Aihara [22] independently proposed an idea of the index of local aromaticity, ILA, and overall index of aromaticity, OIA, based on the counting of the Kekule patterns. However, these concepts were found to be closely related to the sextet polynomial and its derivative as [9]... 

Based on this conclusion one can introduce the concept of the partial electron density and draw its contour map in the plane just above and below, say one Bohr radius, the molecular plane [24, 29, 34]. Again for XVI and XII the results of g3 are given in Fig. 5, which is the contribution of the highest three occupied Huckel MO s. Note that due to the pairing theorem proved by Coulson and Rushbrooke [35] the n-electron densities on all the component carbon 2pn orbitals are the same and the contour map of the conventional electron density cannot differentiate any of the local aromaticity of alternant hydrocarbon molecules. 

In the other two isomers of 159, 3-aza-m-benziporphyrin (161) and 2-aza-m-benziporphyrin (163), the aromatic character is also limited to the local aromaticity of the heterocyclic rings [293], Interestingly, the 2-aza ring system could be... 

It has been shown that a linear relationship between the present RLHEs and the r[ values (normalized benzene character in PPP-MO theory [51]) holds very well. Very recently, Suresh and Gadre [49] suggested a new local aromaticity index based on the molecular electrostatic potential topography (MESP) of the 7t regions of BHs, which is obtainable from ab initio MO calculations. This index was also found to bear linear correlation with the RLHE values. As a result, one... 

It is also instructive to look into the global aromaticity of a BH by averaging the local aromaticity of all hexagon rings. Accordingly, the global aromaticity may be measured by an average ... 

By means of ef we can estimate the energy-effect of the interactions of n-electrons along a particular cycle in a polycyclic conjugated molecule. Thus ef measures the effect of cyclic conjugation caused by an individual cycle on the thermodynamic stability of the respective molecule. We could also say that ef measures the local aromaticity. For the purpose of the present considerations it is important that ef(G, Z) is the effect of an individual cycle Z on the total n-electron energy of the conjugated molecule whose molecular graph is G [56]. 

Krygowski, T.M. and Ciesielski, A. (1995a). Local Aromatic Character of Ceo and C70 and their Derivatives. J.Chem.Inf.Comput.Sci, 35,1001-1003. 

MHP is derived in section 5. From the results obtained with this procedure, we develop a set of simple rules that allow predicting a priori without calculations the existence of vibrational modes that break the MPP (section 6). Finally, we show in section 7 how the nuclear displacements that break the MPP can be a useful indicator of local aromaticity in polycyclic aromatic hydrocarbons. 

The noninvasive nature of NMR spectroscopy combined with the chemical specificity of the NMR method provides direct access to the distribution of various chemical constituents for the histochemistry of plant materials in situ NMR spectroscopy can be used to identify the major constituents, and chemical-.shift imaging can be used to spatially localize them. The latter can be applied to localize aromatics, carbohydrates, as well as water and fat or oil in plant samples. The suitability of many fresh fruits and living plants to be studied by NMR imaging results in a variety of applications in agriculture and food science [Mcc I, Mcc2]. 

This is a local aromaticity index [Dewar and Longuet-Higgins, 1952 Dewar, 1969], defined in the framework of the perturbation molecular orbital (PMO) theory as the energy difference... 

Poater, J., Eradera, X., Duran, M. and Sola, M. (2003a) An insight into the local aromaticities of polycyclic aromatic hydrocarbons and fullerenes. Chem. Bur. /., 9, 1113-1122. 

Discrepancy between common local aromaticity measures in a series of carbazole derivatives. Phys. Chem. Chem. Phys., 6, 314—318. 

This raises the important question of nanotube aromaticity in general. A distinction must be made here between local aromaticity and superaromaticity. The latter arises from ring currents that circuit the perimeter of the respective tube. Figure... 

Anastrozole is a potent and selective triazole AI. It, like letrozole, binds competitively and specifically to the heme of the CYP19. Anastrozole 1 or 10 mg administered once daily for 28 days reduces total body aromatization by 96.7 and 98.1%, respectively. In addition, anastrozole reduces local aromatization in large, ER-positive breast tumors. 

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