Index of aromaticity

Scheme 9.S. Stabilization Energies and Index of Aromaticity for Heteroaromatic Structures Isoelectronic with Benzene or Naphthalene "...

Index of aromaticity based c bond-length variation (benzene = 100). ... 

It is on the value of A that the index of aromaticity p is based (84RRC613)... 

An index of aromatic character based upon statistical evaluation of the deviations in peripheral bond orders has been devised and applied to five-membered ring heterocycles and their mesoionic derivatives including 1,2,3,4-thiatriazole <85TI409> and 5-phenyl-l,2,3,4-thiatriazol-3-oxide <93T8441>. See also <93QSAR146>. 

A new index of aromaticity has been proposed and applied to neutral and mesoionic five-membered heterocycles. The aromaticity index is based on data from experimentally determined bond lengths which yield statistically evaluated bond orders. A reasonable parallel exists between the aromaticity index and resonance energies <85T1409>. 

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]... 

According to the data of Table 2, both energetic and magnetic indexes of aromaticity again advocate that the molecule of 1,2,3,5-thiatriazole is more aromatic than the isomeric 1,2,3,4-thiatriazole and even thiophene. Nevertheless, the parent 1,2,3,5-thiatriazole has not been described in the literature as mentioned before. 

Bird aromaticity index (I) - An index of aromatic character based on a statistical evaluation of the extent of variation of ring bond orders compared to those of the nondelocalized Kekule structure. Bond orders are determined from experimentally determined bond lengths, or from accurate calculated values. The index was introduced for five-membered ring (7S) in 1985 by Clive Bird and subsequently extended to six-membered rings If) and bicyclic systems (7S,6 and 766). A universal index 7a unifies the approach (7a = 76 = 1.235 7s = 1.840 76,6 = 2.085 I tf) <1992T335>. For examples see Sections 2.24.2.3, 2.34.2.3, and 244.2.3. 

Harmonic oscillator model of aromaticity (HOMA) — This is a geometry-based index of aromaticity that takes into account two effects. These are the increase in bond-length alternation (GEO term) and the increase in mean bond length in the system (EN term) such that HOMA= 1-EN-GEO <2004PCP249>. For examples see Sections 2.2.42.3, 2.3.42.3, and 244.2.3. 

Proton chemical shifts are also a potential index of aromaticity. Thus, in pyrazine 7 the protons resonate at S = 8.6 ppm, whereas in its antiaromatic 162 and nonaromatic 163 derivatives the double bond protons show signals at 4.64 and 5.38 ppm, respectively <1983AGE171>. There is qualitative agreement with the ring current influencing... 

As listed in Table 33, a-H s of the heterocycles appear downfield and the difference between a- and / -H s are 0.22 ppm for thiophene. If an assumption is made that the difference of a- and /3-H s of benzene is zero because it is fully aromatic and it corresponds to the index of aromaticity of 1.00, then the relative indices of aromaticity may be related to the difference between and 5p.H- A set of indexes may be proposed for thiophene 0.89 (=1.00 — 0.22/ 2). This seems quite reasonable not only because the difference in chemical shift of a- and / -H is likely to originate from the presence of the heteroatom but also because the values are in the range of reported sets of aromaticity indexes <1974AHC( 17)255, 1993AHC(56)303, 1991H(32)127, 1981JST(85)163, 1985T1409>. 

A series of m- and -disubstituted anilides of thienoic acid and of 2-benzoylthiophenes, which have substituents at the m- and />-position of the benzoyl ring, have been prepared and their IR and NMR spectra obtained in 0.1 M chloroform-r/ solutions and DMSO-r4. The chemical shift values of each series were plotted against the Hammett substituent parameters to give good correlation. The slopes as well as the differences in chemical shift gave a set of meaningful values for the indexes of aromaticity <2002JHC1219>. 

Calculations carried out on all of the molecules of this set (2.335-2.344) allowed predictions to be made as to which should be aromatic." The prediction for each macrocycle is listed in Table 2.3.1, along with the experimentally determined result (where available). This model, which was based on an index of aromatic stabilization per 7t-electron, did not take into account unfavorable effects owing to the larger size of NH and S relative to O." In spite of this obvious oversimplification, in those cases where experimental data are available, these calculations have proved to be remarkably accurate. 

Gautzsch, R. and Zinn, P. (1996). Use of Incremental Models to Estimate the Retention Indexes of Aromatic Compounds. Chromatographia, 43,163-176. 

In the same way as the a, -separation has been performed, one can proceed to a cr, rr-separation.52 This separation has been used to evaluate the aromaticity of organic molecules and clusters. An index of aromaticity was proposed using a scale based on the bifurcation analysis of the ELF constructed from the separated densities. In principle, the total ELF has no information about tt and cr bonds, it depends only on the total density. Hence, the ELF does not show clear differences between both kinds of bonds. However, the topological analysis over separated densities, ones formed by the rr-orbitals and the other ones formed by the cr-orbitals, yields the necessary information.52 Of course, this is possible only for the molecules which present the cr, tt symmetries, i.e. planar molecules. The bifurcation analysis of the news ELFW and ELFCT can be interpreted as a measure of the interaction among the different basins and chemically, as a measure of electron delocalization.45 In this way, the tt and a aromaticity for the set of planar molecules described in the Scheme 1 has been characterized.52... 

Several indexes of aromaticity known from unsaturated hydrocarbons were applied to these metal clusters, apparently succeeding in describing its all-metal aromaticity The geometry of the Al42 unit preserves a square structure (bond length equalization), the global hardness is in the same magnitude as in traditional aromatic molecules, and NICS and DI indexes also support the aromatic character of these molecules. 

As the best available measure of classical aromaticity, Katritzky et al. commended the Bird 4 parameter, which is a new index of aromatic character based on a statistical evaluation of the deviations in peripheral bond orders <8511409, 86T89>, and is later improved to /a <92T335>. Unlike the resonance and aromatic energetical order, the Bird D or 7a estimates that pyrazine has a higher stability than pyridine and other diazines. 

Scheme 8.6. Stabilization Energy and Index of Aromaticity for Heteroaromatic Compounds Isoelectronic with Benzene and Naphthalene... 

AMI Stabilization in kcal/mol relative to localized model using AMI semiempirical calculations M. J. S. Dewar and A. J. Holder, Heterocycles, 28, 1135 (1989). lA Index of aromaticity based on bond length variation. Benzene = 100. 

Two methods are used to determine bromine index of aromatic hydrocarbons which contain trace amounts of olefins and are substantially free of materials lighter than isobutane and have distillation end-point lower than 288 C. The meteods measure trace amounts of unsaturations in materials which have a bromine index below 500. 

ASTM D 5776-98. Standard method for bromine index of aromatic hydrocarbons by electrometric titration. 

Krygowski 5 harmonic oscillator model of aromaticity (HOMA) as structural index of aromaticity S... 

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