Aromaticity structural indices

Recent investigations provide new insight on the structural chemistry of dissolved organic matter (DOM) in freshwater environments and the role of these structures in contaminant binding. Molecular models of DOM derived from allochthonous and autochthonous sources show that short-chain, branched, and alicyclic structures are terminated by carboxyl or methyl groups in DOM from both sources. Allochthonous DOM, however, had aromatic structures indicative of tannin and lignin residues, whereas the autochthonous DOM was characterized by aliphatic alicyclic structures indicative of lipid hydrocarbons as the source. DOM isolated from different morphoclimatic regions had minor structural differences. 

Sander, L.C. and Wise, S.A., Polycyclic Aromatic Hydrocarbon Structure Index, Natl. Inst. Stand. Technol. Spec. Publ. 922. U.S. Government Printing Office, Washington, 1997. 

As shown, for example, by calculations of the structural index A, the aromaticity of alumobenzene (112) is still lower than that in borabenzene. Calculations (MNDO) have shown the ground state of this molecule to be, in contrast to benzene, triplet (3B,). Even so, the 67r-electron structure of (112) is more stable than the 47r-electronic one. However, the difference between their energies (36.6 kcal/mol) is nearly twice as small as that for the corresponding structures of borabenzene. 

According to a calculation of the structural index A (MINDO/3 geometry) [81JOM(215)315], the aromatic character of the recently synthesized... 

The sydnones may be represented by structures (160a-d), of which the zwitterionic structures (160a,b) most clearly imply an aromatic sextet. p-Bromophenylsydnone (160 R-H, R -p-bromophenyl) is essentially planar. However, the O—N bond and 0(1)—C(5) bond lengths are not very different from normal single bond distances. Contributions of structures (160c,d) to the resonance hybrid are important, and the structural index of sydnone reveals its a rather low aromaticity (AN= 17%). 

A further structural parameter which can now be calculated from H/C, H.,r/H, and f is the aromatic substitution index S,r ... 

Figure 9.4 Example substructures with high hit rates. PRI=Promiscuity Ratio Index (see text for explanation). A represents any atom that can form part of an aromatic structure.

The refractive inde.x is measured in a refractometer with a sodium vapor lamp (Na-D lines, 589.0 and 589.6 nm). The value of the refractive index [14.45], [14.46], [14.80] is largely determined by the hydrocarbon skeleton of the substance in question. Aliphatic esters, ketones, and alcohols have refractive indices between 1.32 and 1.42. In homologous series the refractive index increases with increasing length of the carbon chain, and decreases with increasing branching. Cycloaliphatic and aromatic structures increase the refractive index (/Jd ), as does the incidence of functional groups ... 

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. 

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

Sander LC, Wise SA. Polycyclic aromatic hydrocarbon structure index, Natl Inst Stand Tech Spec Publ 922. Washington, DC U.S. Government Printing Office 1997. 

As in the case of density or specific gravity, the refractive index, n, for hydrocarbons varies in relation to their chemical structures. The value of n follows the order n paraffins < n naphthenes < n aromatics and it increases with molecular weight. 

Theoretical and structural studies have been briefly reviewed as late as 1979 (79AHC(25)147) (discussed were the aromaticity, basicity, thermodynamic properties, molecular dimensions and tautomeric properties ) and also in the early 1960s (63ahC(2)365, 62hC(17)1, p. 117). Significant new data have not been added but refinements in the data have been recorded. Tables on electron density, density, refractive indexes, molar refractivity, surface data and dissociation constants of isoxazole and its derivatives have been compiled (62HC(17)l,p. 177). Short reviews on all aspects of the physical properties as applied to isoxazoles have appeared in the series Physical Methods in Heterocyclic Chemistry (1963-1976, vols. 1-6). 

On the basis of data obtained the possibility of substrates distribution and their D-values prediction using the regressions which consider the hydrophobicity and stmcture of amines was investigated. The hydrophobicity of amines was estimated by the distribution coefficient value in the water-octanole system (Ig P). The molecular structure of aromatic amines was characterized by the first-order molecular connectivity indexes ( x)- H was shown the independent and cooperative influence of the Ig P and parameters of amines on their distribution. Evidently, this fact demonstrates the host-guest phenomenon which is inherent to the organized media. The obtained in the research data were used for optimization of the conditions of micellar-extraction preconcentrating of metal ions with amines into the NS-rich phase with the following determination by atomic-absorption method. 

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

As follows from Table I (see Section VII Index of Tables), the yields in 4-acetylenyl compounds depend both on the reaction time and on the structure of the aromatic and acyclic components (molecular polarity). If more than one equivalent of diazomethane is used, N-methylation of pyrazole occurs. 

The n-d-M correlation is an ASTM (D-3238) method that uses refractive index (n), density (d), average molecular weight (MW), and sulfur (S) to estimate the percentage of total carbon distribution in the aromatic ring structure (% C ), naphthenic ring structure (Cj,), and paraffin chains (% Cp). Both refractive index and density are either measured or estimated at 20°C (68°F). Appendix 4 shows formulas used to calculate carbon distribution. Note that the n-d-M method calculates, for example, the percent of carbon in the aromatic ring... 

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