Kekule structures, benzenoids

This concept of Kekule structures can be extended to the 3D deltahedral borane anionsB H " (6Kekule structures make use of three-center B-B-B bonds instead of the carbon-carbon double bonds in benzenoid Kekule structures. Lipscomb s semitopological method"" " for studying the electron and orbital balance in boron networks containing mixtures of B-B two-center and B-B-B three-center bonds is essential for extending the concept of Kekule structures from 2D benzenoid hydrocarbons to 3D deltahedral boranes. 

Chen, R. S., Cyvin, S. J., Cyvin, B. N., Brunvoll, J., and Klein, D. J. Methods of Enumerating Kekule Structures, Exemplified by Applified by Applications to Rectangle-Shaped Benzenoids. 153, 227-254 (1990). 

This isomer represents the first example of a [5,6]-adduct in which the fusion bond remains intact. This addition mode was explained by the relatively high double bond character of the [5,6]-bond C(7)-C(8), which underlines the importance of the Kekule structure of C7Q that involves the benzenoid hexagons at the equatorial belt. No such resonance structure is required to describe the proper bonding situation within C50 (see also Chapters 1 and 14). 

The following bonds occur in benzenoid hydrocarbons. Consider first the endocyclic carbon-carbon bonds, namely, those found in benzene, with = 115.39 (or Sqq = 124.84) kcal/mol, which—in a sketchy way—are some kind of averages between a single and a double sp -sp bond. (Their number is double that of the number of double bonds that can be written in classical Kekule structures, e.g., 2 X 5 in naphthalene, 2 x 7 in anthracene.) But in polynuclear benzenoid structures there are not twice as many averages as there are Kekule double bonds. Hence, consider the extra single C sp )—C sp ) bonds like the one found in naphthalene, or the two extra single bonds found in anthracene. The appropriate bond energy formulas are... 

Zhang FJ, Guo XF, Chen RS (1990) The Existence of Kekul Structures in a Benzenoid System. 153 181-194... 

Note that when the Clar convention is used for Kekule structures, the benzenoid rings are often shown as circles (e.g., see Moyano and Paniagua, 1991).]... 

Chang CWJ, Scheuer PJ (1993) Marine Isocyano Compounds. 167 33 - 76 Chen RS, Cyvin SJ, Cyvin BN, Brunvoll J, Klein DJ (1990) Methods of Enumerating Kekull Structures. Exemplified by Applified by Applications of Rectangle-Shaped Benzenoids. 153 227-254... 

Cyvin SJ, Gutman I (1988) Kekule structures in benzenoid hydrocarbons, Springer, Berlin Heidelberg New York... 

Algorithms A and C enable the number of Kekule structures and Pauling s bond orders to be determined and the characteristic polynomial and the eigenspaces (eigenvalues, eigenvectors) to be calculated for all benzenoid systems. 

The Literature on Benzenoid Hydrocarbon Calculations 2.1 Kekule Structures and Pauling s Bond Orders... 

Let K = K(B) denote the number of Kekule structures contained in a benzenoid system B. 

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