Kekuld structure

Fig. 29. Localized 7T-MO s in catacondensed molecules for which the LMO s do not correspond to Kekuld structures. Fifth strongest contour is shown... 

The resonance energy for the two Kekuld structures in benzene can be calculated in terms of a by neglecting all interactions except those... 

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

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

The Kekuld wave function also yields considerable insight into the structure of buckminsterfullerene. Since buckminsterfullerene is a Clar sextet molecule, it has the special Kekule structure which places double bonds on all 30 of the 6-6 edges. Since the pi electrons tend to localize to these positions, this Kekule function should be the most important single contributor to the ground state wave function, while Kekuld functions with many double bonds in 6-5 positions should be less important. Fig. 3 shows the absolute value of the wave function coefficient for each Kekuld structure as a function of the number of double bonds in 6-5 positions. The expected decreasing trend is clearly observed, but something else stands out as well. The Kekule functions divide into two classes, separated by the solid line in the figure. This separation is a reflection of the nonaltemant character of a fiillerene. 

The molecular orbital answer to this problem, as you may well know, is that all six p orbitals can combine to form (six) new molecular orbitals, one of which (the one lowest in enery ) consists of a ring of electron density above and below the plane of the molecule. Benzene does not resonate between the two Kekuld structures—the electrons are in molecular orbitals spread equally over all the carbon atoms. However the term resonance is still sometimes used (but not in this book) to describe this mixing of molecular orbitals. 

Chlorobenzene is considered to be a hybrid of not only the two Kekuld structures, I and II, but also of three structures. 111, IV, and V, in which chlorine is... 

Figure 3.6. Mesomeric Kekuld structures of the (a) cyctopentadimyl radical, (b) cyclopentadienyi anion, and (c) cyclopentadienyt cation. ...

A generalization of Eq. (97) was proposed i0 i87)( which shows the dependence of the number of Kekuld structures on molecular topology for arbitrary hydrocarbons. The Sachs theorem is used in order to derive the following equation ... 

The two Kekuld structures for benzene have the same arrangement of atoms, but differ in the placement of electrons. Thus they are resonance forms, and neither one by itself correctly describes the bonding in the actual molecule. As a hybrid of the two Kekule structures, benzene is often represented by a hexagon containing an inscribed circle. 

The extent to which benzene is more stable than either of the Kekuld structures is its resonance energy, which is estimated to be 125-150 kJ/mol (30-36 kcal/mol) from heats of hydrogenation data. 

The dissociation of hexaphenylethane leads to the formation of two triphenylmethyl radicals. With the central carbon atom in the trivalent state eight Kekuld structures are possible the nine carbon atoms in the ortho and para positions in the three rings may also be in a trivalent state and thus in all, 36 valence bond structures are possible for the free radical. As a result of the resonance among these structures there is a gain in energy of the free radical by 50 kcals, with the result that the dissociation energy of hexaphenylethane is as low as 11-12 kcals. 

Figure 22. Two fully naphthalenoid structures. [But, following Knop et al. (Ref 69) one would regard only (a) as being truly" fully arenoid, because in (b), Kekuld structures can be drawn that involve the empty hexagons.]...

Deltahedron Formula Degeneracies Existence of Kekuld structure for B H ... 

Note that if the graph G represents a conjugated molecule with n conjugated centers (n = even), m(G,n/2) is equal to the number of Kekuld structural formulas of the respective mdecule. This is a quantity of crucial importance in rescmance theory [62]. 

Kekuld structure count and algebraic structure count... 

The Resonance Representation The resonance picture of benzene is a natural extension of Kekuld s hypothesis. In a Kekuld structure, the C—C single bonds would be longer than the double bonds. Spectroscopic methods have shown that the benzene... 

The resonance-delocalized picture explains most of the structural properties of benzene and its derivatives—the benzenoid aromatic compounds. Because the pi bonds are delocalized over the ring, we often inscribe a circle in the hexagon rather than draw three localized double bonds. This representation helps us remember there are no localized single or double bonds, and it prevents us from trying to draw supposedly different isomers that differ only in the placement of double bonds in the ring. We often use Kekuld structures in drawing reaction mechanisms, however, (o show the movement of individual pairs of electrons. 

Only one Kekuld structure is shown for each compound.)... 

---