Kekule-like structures

As we continue to larger rings, the results are not so clearcut. There is a tendency for the saddle points to alternate in and out somewhat, but the interaction energy appears to be a maximum at the ring of six atoms. These last more-or-less qualitative comments have been based upon just the two simple Kekule-like structures of the rings, and may not be able to show the proper behavior. The actual surface calculations of ( 2)2 and (H2)3 included many more structures and show strikingly different qualitative behavior. 

Attempts to localize the double bonds of the benzene ring in a Kekule-like structure have been made with some success by the groups of Vollhardt [57] and of Frank and Siegel [58]. This was reminiscent of the Mills-Nixon effect predicted in the 1930 s, although the partial localization has nothing to do with the tautomeric equilibrium in benzene argued by Kekule. 

For the double bonds to be completely conjugated, the annulene must be planar so the p orbitals of the pi bonds can overlap. As long as an annulene is assumed to be planar, we can draw two Kekule-like structures that seem to show a benzene-like resonance. Figure 16-3 shows proposed benzene-like resonance forms for cyclobutadiene and cyclooctatetraene. Although these resonance structures suggest that the [4] and [8]annulenes should be unusually stable (like benzene), experiments have shown that cyclobutadiene and cyclooctatetraene are not unusually stable. These results imply that the simple resonance picture is incorrect. 

The structural preference for the n-bonds is not as clear. Berry noted that the antisymmetric C-C stretching mode, which effectively interconverts Kekule type structures, had an unusually low frequency, and concluded that the n-system preferred a Kekule-like structure.29 Similar conclusions have been derived from... 

The resonance energy of pyridine, relative to one of the Kekule-like structures, is 180 kJ mole". Pyridine is a base in acidic solution it adds a proton to the unshared electron pair of the nitrogen atom, forming the pyridonium ion, C5H5NH. ... 

The concept of electronic delocalization has germinated in the pre-electron period to Kekule s structural theory and its application to benzene as a prototype of a family of compounds so-called aromatics . Kekule had to address two major properties of benzene revealed from substitution experiments. The first was the empirical equivalence of all positions of benzene, what is called today the Dfjh symmetry of both geometry and electronic structure, and second the persistence of the aromatic essence in chemical reactions, what we recognize today as aromatic stability . Thus, Kekule postulated that there is a Ce nucleus and the four valences of the carbons are distributed to give two oscillating structures, which when cast in our contemporary molecular drawings look like part a in Scheme 2.39-44 One of the many alternative hypotheses on the nature of... 

An attempt at identifying the products of nitroindazole methylation was unsuccessful [314], The authors failed to distinguish the kekule -like (1-methyl) (I) and quinoid -like (2-methyl) (II) structures [651] ... 

Benzene undergoes substitution rather than addition, Kekule s structure of benzene is one that we would call cyclohexatriene. We would expect this cyclohexatriene, like the very similar compounds, cyclohexadiene and cyclohexene, to undergo readily the addition reactions characteristic of the alkene structure. As the examples in Table 10.1 show, this is not the case under conditions that cause an alkene to undergo rapid addition, benzene reacts either not at all or very slowly. 

Solving the simultaneous equation (19) leads toy = 3 and t = n - 2 implying the presence of three two-center B-B bonds and - 2 three-center B-B-B bonds. Since a deltahedron with n vertices has 2n - 4 faces, the n - 2 three-center B-B-B bonds will cover exactly half of the faces. In that sense a Kekule-type structure for the deltahedral boranes B H has exactly half of the faces covered by three-center B-B-B bonds just like a Kekule structure for a benzenoid hydrocarbon has half of the edges covered by C=C double bonds. 

The reason that MO theory has advanced so much is that it is relatively easy to compute corresponding matrix elements in MO models. For example, if the MO method is applied to a molecule like buckmin-sterfullerene, one would consider overlapping among 60 a -electrons. In contrast, in the simple VB approach, the resonance theory, one has to consider the 12 500 Kekule valence structures that this... 

Because Kekule valence structures are central to the concept of conjugate circuits, we will follow with a discussion of the properties of Kekule valence structures, their construction, and their enumerations. We will be relatively brief and would like to direct the attention of readers to the book by Cyvin and Gutman on Kekule structures in benzenoid hydrocarbons, where the enumeration and properties of benzenoid structures are discussed on over 300 pages. Kekule valence structures have been around... 

Figure 75. Benzenoid-like structures that represent fragments of a graphite lattice but have no Kekule valence structures.

Figure 76. So-called concealed benzenoid-like structures (having an equal number of starred and non-starred carbons) that have no Kekule valence structures. " ...

Numerical Kekule Valence Structures. Kekule valence structures, like Clar s valence structures, may be viewed as geometrical formulas because they are drawn. Similarly, the novel notation for benzenoid hydrocarbons using an inscribed Greek jt represents a pictorial, qualitative, non-numerical representation of such molecules. However, well-designed or selected notation may have advantages that initially one did not anticipate. This has been the case with the Leibniz notation for derivatives in mathematics, and with graphical codes of configuration of /7-alkanes in structural chemistry. We will show now that the just introduced qualitative Jt-nota-... 

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