Aromaticity structural measures

Aromaticity, defined as a structural feature, was used as a predictive tool for compounds that had not been prepared previously, whereas reactivity, bond length, or magnetic criteria had to await the isolation of a compound and its experimental investigation. Only recently has the development of quantum-chemical methods reached the point where one can predict with sufficient accuracy the magnetic properties, the bond lengths, and the reactivity patterns of aromatics. The multidimensional character and the definition and measurement of aromaticity generated confusion and conflicts.43 A recent review discussed the multidimensional character of aromaticity and theoretical and experimental approaches to aromatic structures and their predictions, and references are indicated extensively.66... 

Reduction apparently creates fresh hydroaromatic structure (at the expense of the aromatics), and thus the methyl groups attached to the aromatic structures are likely to become amenable to quantitative estimation (by the Kuhn-Roth procedure), provided that the particular aromatic ring is reduced to hydroaromatic. Significantly, the reduced samples of the lower rank coals did not yield much higher values for methyl groups than the original samples. Thus, it would appear that Kuhn-Roth estimation does not completely measure the true C-methyl content in coals, especially in high rank coal samples. 

Typically, 40—50% of the carbon atoms in lignite are in aromatic structures while 60—70% of the carbon atoms in Illinois bituminous coal are in aromatic structures (7,8). By all of these measures, waste fuels are significandy more reactive than coal, peat, and other combustible solids. 

The TB MO calculation on the 15N chemical shift of polypyrrole in the solid state allows useful information to be extracted from the observed spectra, namely that the two peaks obtained are correctly assigned to the quinoid and aromatic structures.(l 1,38) ( The quinoid structure is closely to the electric conductivity.) A decrease in the band gap leads to a downfleld shift. These results on conducting polymers demonstrate that the chemical shift behavior provides information about the band gap which, in turn, is a measure of the electric conductivity. It can be said that TB MO calculations offer useful perspectives in interpreting the results of NMR nuclear shieldings in polymers, both in terms of the structure in the solid state and in understanding the effect of intermolecular interactions on nuclear shieldings. The latter are shown to operate through the electronic structures of the polymers considered. 

The methods used for complete structure determination cannot be reviewed in any detail here. It is well known that the central problem is that of determining the relative phase relations of the diffracted waves. As only the intensities can be observed, the problem may be approached by trial and error calculations until a good measure of agreement with the observed values is obtained. This method has in fact been used for a great many of the aromatic structures dealt with in this article. The rigid molecular frameworks with at most only a few degrees of freedom lend themselves to such a treatment. A more systematic approach along the same lines is possible by the method of Fourier transforms (Lipson and Taylor, 1958). 

Sulfur compound preferred to hydrocarbon because of precise mass measurement (0.000 error for C22H14S, 0.0034 for C25H10) and because no reasonable aromatic structure exists for a C25H10 hydrocarbon. 

An examination of the photoelectron (PE) spectra of 42 five-membered heteroaromatics and simple derivatives thereof, including furan and furan derivatives, showed only minor perturbations of the aromatic structures by the substituents <1993MI811>. Threshold PE spectra as well as photoabsorption and photoion spectra of furan and furan-, 4 have been measured and interpreted <1998CPH(236)365, 2001CPH(263)149>. 

Flame retardants may not only catalyze dehydration of the cellulose to more char and fewer volatiles but also enhance the condensation of the char to form cross-linked and thermally stable polycyclic aromatic structures (60). Cellulose was treated with various additives and then charred at 400 °C. The chars were then oxidized with permanganate see Chapter 13) and the results are in Table IV. The char yield was slightly higher for the sodium chloride-treated sample (17.5%) and substantially more for the sample containing diammonium phosphate (28.9%), as compared to the yield from the untreated sample (15.3%). Furthermore, the increased char formation was accompanied by increased aromaticity, as measured by the amount of the aromatic carbon obtained from the char and the amount obtained from the original cellulose molecules (60). 

Voacoline (VII), pA 5.70, a minor component of V. chalotiana, is a sarpagine derivative which contains a hemiketal moiety (37). This functionality accounts for the ease with which voacoline forms 0-methyl (mp 211°-212°) and 0-ethyl (mp 215°-216°) derivatives (VIII) (see analogous properties of pseudostrychnine and eburnamine). The structure of voacaline has not been proved unequivocally, but all the available evidence is in its favor. The methyl functions were identified by NMR-spectroscopy iwd-N-Me(3.6ppm), COOMe (3.0 ppm, partially shielded), — Me (4.3 ppm three proton singlet). There was also a —CH2—0— (3.7 ppm singlet two protons) and four aromatic protons. Measurement of the NMR-spectrum of a compound in dimethylsulfoxide permits the coupling between the hydroxyl proton and the protons on the carbon to... 

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 ... 

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