Alternant hydrocarbons classification

An important distinction for conjugated hydrocarbons is the classification into alternant and non-alternant hydrocarbons. Alternant hydrocarbons are those like ethene, hexatriene, benzene and naphthalene where we can divide the carbon atoms into two sets called starred and unstarred , such that no member of one set is directly bonded to another member (Figure 7.4). 

Conjugated hydrocarbons that do not contain an odd-membered ring are called alternant hydrocarbons (AHs). The distinction between alternant and non-alternant hydrocarbons (NAHs) provides a very important classification of conjugated hydrocarbons, especially with regard to excited states. In AHs, the unsaturated C atoms can be assigned to two sets, the starred ( ) and the unstarred (o) set, such that no atoms of the same set are bound to each other. This is not possible for NAHs (Figure 4.18). 

Hence, naphthalene, phenanthrene, pyrene, and biphenylene are alternant hydrocarbons, while azu-lene, pyracylene, and its isomers are non-alternant conjugated hydrocarbons. The idea of classification of hydrocarbons as alternant and non-alternant goes back to Coulson and Longuet-Higgins and the early days of HMO theory. An important property of alternant hydrocarbons, as opposed to non-alternant, is that for alternant systems all HMO eigenvalues come in pairs Xu while this is not the case for nonalternant systems. It is interesting to mention that this particular mathematical property of alternant... 

Transition metal catalysts, specifically those composed of iron nanoparticles, are widely employed in industrial chemical production and pollution abatement applications [67], Iron also plays a cracial role in many important biological processes. Iron oxides are economical alternatives to more costly catalysts and show activity for the oxidation of methane [68], conversion of carbon monoxide to carbon dioxide [58], and the transformation of various hydrocarbons [69,70]. In addition, iron oxides have good catalytic lifetimes and are resistant to high concentrations of moisture and CO which often poison other catalysts [71]. Li et al. have observed that nanosized iron oxides are highly active for CO oxidation at low tanperatures [58]. Iron is unique and more active than other catalyst and support materials because it is easily reduced and provides a large number of potential active sites because of its highly disordered and defect rich structure [72, 73]. Previous gas-phase smdies of cationic iron clusters have included determination of the thermochemistry and bond energies of iron cluster oxides and iron carbonyl complexes by Armentrout and co-workers [74, 75], and a classification of the dissociation patterns of small iron oxide cluster cations by Schwarz et al. [76]. 

According to Hiickel s rule, annulenes with 4n tt electrons are not aromatic. Cyclobutadiene and cyclooctatetraene are [4n]-annulenes, and their properties are more in accord with their classification as cyclic polyenes than as aromatic hydrocarbons. Among higher [4n]-annulenes, [16]-annulene has been prepared. [16]-Annulene is not planar and shows a pattern of alternating short (average 134 pm) and long (average 146 pm) bonds typical of a nonaromatic cyclic polyene. 

We introduce here for the conjugated hydrocarbon radicals the same classification which we used for the closed-shell hydrocarbons 55). We divide the hydrocarbons into two large groups alternant and nonalternant. Further classification concerns the even and odd systems, and the presence of cycles in the skeleton. The phenyl substituents are... 

Petroleum ether ("pet ether") is a mixture of hydrocarbons formulated to have a boiling point between 35 and 80°C. Pet ether contains no ethers. It is also called naptha, benzin, or petroleum benzin. Since the mixture varies, constants depend on the batch and the manufacturer. The solvent group is a classification based on similarity of behavior and solvent properties. It is useful for making generalizations and for selecting alternative solvents when safety, cost, or other constraints apply. 

The main reason to mention perfect Clar structures is to draw the attention of readers to different definitions used here by Fowler and Pisanski, differ ent classifications of Clar cages by Flocke, Schmalz, and Klein, and different definitions of Clar structures of fullerenes as we have outlined at the beginning of this section. Upon closer examination, we expect readers to agree at our approach is the only one that truly extends the spirit of the jt-aro-matic sextets of Clar from benzenoid hydrocarbons to fullerenes. That does not make the alternative approaches, which are equally legitimate, less worthy, as each of them serves its own purposes — but readers should be alerted in advance and should be spared possible confusion due to use and promotion of the name of Clar in several different conceptual connections. Observe that in our definition of the Clar structure of fullerene, only hexagonal faces can be the site of aromatic jr-sextets, but that is not the case in the approach of Fowler and Pisanski, where pentagonal faces play a role similar to that of hexagonal faces. 

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