Measurements of aromaticity

Chemical Composition. Chemical compositional data iaclude proximate and ultimate analyses, measures of aromaticity and reactivity, elemental composition of ash, and trace metal compositions of fuel and ash. All of these characteristics impact the combustion processes associated with wastes as fuels. Table 4 presents an analysis of a variety of wood-waste fuels these energy sources have modest energy contents. 

The concept of relative hardness and topological resonance energy per electron as a measure of aromaticity, with a relative hardness value of zero as the border for antiaromaticity, failed for 1-benzothiepin,3 2... 

Ab initio electron correlated calculations of the equilibrium geometries, dipole moments, and static dipole polarizabilities were reported for oxadiazoles <1996JPC8752>. The various measures of delocalization in the five-membered heteroaromatic compounds were obtained from MO calculations at the HF/6-31G level and the application of natural bond orbital analysis and natural resonance theory. The hydrogen transfer and aromatic energies of these compounds were also calculated. These were compared to the relative ranking of aromaticity reported by J. P. Bean from a principal component analysis of other measures of aromaticity <1998JOC2497>. 

To perform a similar evaluation for the indenyl cation, 18, we must examine the bis-indenylidene dication, 19, and the appropriate data for both systems is given in Table IV. While 19 is more antiaromatic than 18 by all measures, the dependence of the measure of aromaticity and presumably antiaromaticity on ring size requires that the values for 19 be divided by 2. When examined in this manner, the indenyl cation 18 is more antiaromatic than 19, in contrast to the behavior of the fluorenyl system in 7 and 3. 

That is, just as oxidation of tetrabenzo[5.5]fulvalene resulted in formation of two antiaromatic fluorenyl cations linked by a single bond, reduction should give two aromatic fluorenyl anions, linked by a single bond, 20. We call this relationship the aromaticity/antiaromaticity continuum and can apply the same measures of aromaticity to 20 as were applied to 3, to evaluate relative aroma-... 

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... 

The aromaticity of five-membered rings with two or more heteroatoms was discussed in detail in earlier reviews.52 100 111 In a comprehensive survey on the quantitative measurements of aromaticity,112 it has been shown that basicity-based quantification of aromaticity gave more reproducible resonance energies than other methods, such as heats of formation, ring currents, magnetic susceptibilities, and theoretical indices. 

Generalized valence bond interaction energies were computed for mono/poly-nitrogenous five- and six-membered heterocycles.203 Results that diverged from those obtained by other methods were obtained only for poly-nitrogenous systems such as pyridazine, benzotriazole, and tetrazole, which may confirm Bird s earlier finding123 204 that electron delocalization is not a stand-alone and direct measure of aromaticity for nitrogenous heterocyclic compounds. 

Wang, J., and Ghen, G. (2003). Microchip capillary electrophoresis with electrochemical detector for fast measurements of aromatic amino acids. Talanta 60, 1239—1244. 

Finally, if we abandon Hiickel s topological approach altogether and consider more elaborate quantum-mechanical approaches, the concept of aromaticity derived purely from a consideration of -electrons becomes blurred and tends to disappear completely. In fact, allelectron methods allow the calculation of aromatic properties (Section V,B) of a given substance without introducing explicitly the concept of aromaticity. Certain authors, notably Dewar,19 have published resonance energies derived from self-consistent field molecular-orbital (SCF-MO) calculations, and these could be used as a measure of aromaticity. 

Magnetic rotation (Faraday) effects have been little used for assessing aromaticity. Furan has only a very weak effect, and when allowance is made for steric strain in the ring the exaltation, Ep> which can be considered to be a measure of aromaticity, is —0 compared with 45 for pyrrole, 52 for thiophene and 182 p,r for benzene (69Mi3iooi). 

There are several compounds that possess some measure of aromatic character typical of benzene, but do not possess a benzenoid ring. Appropriately, they have (4n + 2) it electrons and are classified as nonbenzenoid aromatic compounds (see Section 21-9). An example is azulene, which is isomeric with naphthalene and has a five- and a seven-membered ring fused through adjacent carbons ... 

---