Descriptor aromaticity descriptors

The numerous aromaticity measures proposed in recent decades have widened the number of descriptors in the literature considerably [2,8,9,44,45], Aromaticity descriptors can be classified as magnetic, energetic, electronic, and structural [46]. 

A set of n = 209 polycyclic aromatic compounds (PAC) was used in this example. The chemical structures have been drawn manually by a structure editor software approximate 3D-structures including all H-atoms have been made by software Corina (Corina 2004), and software Dragon, version 5.3 (Dragon 2004), has been applied to compute 1630 molecular descriptors. These descriptors cover a great diversity of chemical structures and therefore many descriptors are irrelevant for a selected class of compounds as the PACs in this example. By a simple variable selection, descriptors which are constant or almost constant (all but a maximum of five values constant), and descriptors with a correlation coefficient >0.95 to another descriptor have been eliminated. The resulting m = 467 descriptors have been used as x-variables. The y-variable to be modeled is the Lee retention index (Lee et al. 1979) which is based on the reference values 200, 300, 400, and 500 for the compounds naphthalene, phenanthrene, chrysene, and picene, respectively. 

Falque, E., Silva Ferreira, A. C., Hogg, T., and Guedes de Pinho, P. (2004). Determination of aromatic descriptors of Touriga Nacional wines by sensory descriptive analysis. Flavour and Fragr.. 19, 298-302. 

Chemical descriptors are in most of the cases obtained with equations that are not known. Even if the references to certain general equations are given, in practice, it is difficult to replicate the results obtained with chemical descriptors. As we have discussed, chemical descriptors based on tridimensional structures are subject to manual optimization, and this may change the descriptor values. But even in the case of other simpler descriptors, we found that using software from two different commercial sources, the results may be different. Even the use of two different versions of the same software may provide different results for the same descriptor. Even descriptors, which seem simple, such as number of double bonds, or of aromatic rings, are critical because they depend on how tautomers and aromaticity are considered in the different software, or are sensitive to the structure format that is used. 

Iha and Ihd parameters - hydrogen-bonding descriptors aromaticity indices resonance indices... 

In order to compare the global hardness as an all-metal aromaticity descriptor, we determine other quantities employed to evaluate aromaticity. The delocalization index (DI, 8 in Equation i)35,36 is derived from Bader s atoms-in-molecules (AIM) theory37 from the exchange-correlation density matrix (Txc) ... 

Let us briefly analyze the molecular orbitals (MOs) of Al42. It can be seen that, as expected, these MOs are symmetrically quite different from the analogous traditional aromatic cyclic hydrocarbons (Figure 2). This shows that the concept of aromaticity in the all-metal clusters is certainly an extension of the original concept and is not exactly similar. This has been already discussed in the introductory section, but it is important to pinpoint that although other aromaticity descriptors (hardness, DI, and NICS) may go... 

Results for the complexes with general formula MAl4n where M is a cation from group I and II Other aromaticity descriptors—DI and NICS... 

Some correlation between NICS index and HOMA index was found out [Szatylowicz, Krygowski et al., 2007]. However, criticism about the use of NICS as aromaticity descriptor is made by Lazzeretti [Lazzeretti, 2004], concluding that. a quantitative theory of aromaticity based on NICS is epistemologically inconsistent. ... 

Figure 15. Interatomic distances in H-bonds (A, in parentheses), their energies (kcal/mol) of interactions, and estimated aromaticity descriptors for the /j-nitrosopheno-iate anion in two salts sodium and magnesium, respec-tiveiy. (Reprinted with permission from ref 47. Copyright 1998 Elsevier Science.)...

For one and the same benzenoid with 11 rings of six different types, namely tribenzo[a,/, rs ]phenanthra[l,2,10-c /e]pentaphene (10/15) with 11 benzenoid rings (Krygowski et al. 1995 Oonishi et al. 1992) the data in Table 8.4 indicate fair correlations between EC and various local aromaticity descriptors, albeit with only six points. Thus for EC and GT, = 0.926) for EC and HOMA, = 0.986 for EC and the algebraic Clar structure index (Randic 2011), = 0.864. 

Outline of the Most Frequently Used Aromaticity Descriptors... 

OUTLINE OF THE MOST FREQUENTLY USED AROMATICITY DESCRIPTORS... 

Following the conclusion of Cyranski et al. (2002JOC1333), the computed correlations between various aromaticity descriptors for subgroups (aromatic, nonaromatic, and antiaromatic) are much worse than for the whole set of data (see Supporting Information by Alonso and... 

Here we have described the most popular aromaticity descriptors applicable to heteroaromatic systems and have considered possible correlations between them for five- and six-membered heterocycles. The relationship of aromaticity with stability of tautomers, substituent effects and intermolec-ular interactions in the case of nucleobases have been also discussed. 

The bond alternation coefficient (BAG) has also been proposed as an aromaticity descriptor which has been defined as... 

Up to now, essentially aromaticity descriptors for starting and final products have been discussed. In... 

The application of quantitative methods for the evaluation of the molecular aromaticity is common (Bird 1992 Schleyer et al. 1996 Fores et al. 1999 Cyranski et al. 2002 Bultinck et al. 2005 Espinosa et al. 2005 Mitchell et al. 2005 Zborowski et al. 2005 Shishkin et al. 2006 Tarko 2008a Tarko 2010) however, the listing of aromaticity descriptors has emerged recently (Tarko 2008a Tarko 2010). 

The number of aromatic bonds minimum, average, and maximum value of the aromaticity of the aromatic bonds percentage of aromatic bonds number of aromatic molecular zones or aromaticity of the peripheral topological path can be calculated as descriptors. Some descriptors simultaneously reflect the aromaticity and size of the molecule. Other descriptors reflect the concentration of aromaticity in small areas of the molecule. 

When the values of aromaticity descriptors have been obtained, the application of QSPR7QSAR methodology makes possible the verification of the dependence of some macroscopic properties (magnetic, electrical, optical, biochemical) on aromaticity (Tarko 2010). 

Eventually, we proposed an our own index based on the SF descriptor (SFLAI, Source Function Local Aromaticity Index) for quantifying the degree of aromaticity of 6-membered rings (6MRs) in polycyclic systems. Analogously to the SF analysis of electron delocalization, such an index might prove to be particularly useful for application to experimentally-derived ED s, as, at variance with other commonly employed quantum-mechanical (local) aromaticity descriptors, it does not require the knowledge of the pair density. 

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