Anti-Kekule structure

Overlapping could, of course, take place, 1,2 3,4 5,6 or 1,6 5,4 3,2, leading to formulations corresponding to the Kekule structures 4a and 46) but, as an alternative, all six adjacent p orbitals could overlap, as with conjugated dienes (p. 12), resulting in the formation of six molecular orbitals, three bonding 3) and three anti-bonding... 

We have applied Pauling s theory to the molecular hydrogen cluster as follows the nonmetallic cluster is well described by the usual Kekule structure (1-2 3-4) (Fig.2), where orbitals 1 and 2 are at one hydrogen molecule and 3 and 4 are at the other one. The synchronized resonance is the mechanism in which the system alternates between structures (1-2 3-4) and (1-4 2-3) (anti-Kekule) (Fig.2), breaking simultaneously the two original covalent bonds and forming two new ones. 

The valence orbitals (1, 2, 3 and 4) and the metallic orbitals (1 2 , 3 and 4 ) are optimized separately, using a VB calculation with just one structure, namely, the Kekule structure for the former and the anti-Kekule for the later. It is done for each intermolecular separation a (Fig.l), which varies from 1.5 A to 6.0 A. The molecular bond distance d was kept fixed to 0.74 A for all intermolecular separations. We verified that if d is allowed to relax, at the SCF level, it varies at most by a few hundredths of an angstrom and the energy lowers by about 10-4 Hartree. It therefore does not affect our results. Once the orbitals are obtained, we form the 14 structures and solve the VB secular equation. 

The first two structures are famous Kekule structures, the next three are Dewar structures, the sixth is an example of the possible mixed covalent-ionic structures. From these graphs, we may deduce which atomic orbitals (out of the 2p, orbital of carbon atoms, z is perpendicular to the plane of the benzene ring) takes part in the covalent bond (of the tt type). As far as the mathematical form of the Heitler-London functions (involving the proper pairs oflpz carbon atomic orbitals), the first for electrons 1,2, the second for electrons 3,4, and the third for 5,6. Within the functions d>/, the ionic structures can... 

Benzene displays a symmetric Cs symmetry) S /So conical intersection between states that correlate with the E2g state 2 in the FC region) and the ground state of the planar equilibrium structure. hS9-92 Scheme 12, we show that the electronic character of the anti-aromatic (i.e. difference of the two Kekule structures indicated with a dashed circle) Si surface B2u) changes along the reaction path due to an avoided crossing between the S2 and Si states. The S2 CE2g) state can be described by a combination of quinoid (Dewar type) and antiquinoid spin couplings. As seen in Fig. 8(a)... 

Hence, in the case of benzo[ /2i]perylene, the last (the anti-Fries) Kekule valence structure of has df =. The reader may try to determine the dffor the other Kekule valence structures of benzo[ / i]perylene and will immediately see that this may not be an easy task. In the next paragraph, we have listed devalues for the remaining Kekule structures of henzo[ghi -perylene to assist those who tried to find them on their own to verify their findings. 

It suffices to assign C=C type to any of the vertical CC bonds in such molecules, and the bond types of all other CC bonds will be determined. Of the five Kekule valence structures in phenanthrene shown in Figure 18, only the last (anti-Fries) structure has df = 1. The remaining four Kekule structures of phenanthrene have df = 2, because a selection of C=C bond in one of the terminal benzene rings can in no way influence the selection of C=C bonds in the other terminal ring. In Figure 20 we have illustrated some Kekule valence structures having different d/values for a selection of small benzenoid hydrocarbons. 

Before each of the classes is fully described, let us explain why we have /outclasses and not two classes (aromatic and anti-aromatic), or why three classes (aromatic, non-aromatic, and anti-aromatic) will not suffice. There are no problems with the dichotomy aromatic—anti-aromatic based on the presence of only 4n + 2 or only An conjugated circuits in the set of Kekule valence structures of a compound, respectively. The problem is with the non-aromatic class, which would include structures having both An+ 2 and An conjugated circuits. Some structures in this class may show a greater similarity with benzene, and on the other hand they may show some similarity to anti-aromatic compounds. The problem is that the class of such neither aromatic nor anti-aromatic structures, that have both 4n -I- 2 and An conjugated circuits, is so large and so broad that it becomes of little use. 

The theoretical models start with Kekule s [44] description of benzene, as having two structures. Later Hiickel [45,46] discovered his [4 +2] and [4n] rules, and was able to account for the stability of benzene ([4 +2]) and the instability of cyclobutadiene and cyclo-octatetraene (both [4 ]). The [4 +2] compounds were called aromatic after benzene, while the [4n compounds were given the designation anti-aromatic. 

The ontz-isomer (XXXIXB) is sensitive to oxygen, but can be kept under nitrogen [181], whereas the st/w-isomer (XXXIX/l) forms stable orange needles [182]. Electronic and n.m.r. spectra of the syn--isomer show that it has a delocalised electron system around the periphery [182], whereas spectra of the anti-isomer indicate that it is a mixture of two localised valence isomers, corresponding to the two Kekule forms [181]. Variable temperature n.m.r. studies of the latter compound show that the two forms are interconverting this must presumably involve a delocalised transition state, which in turn implies that the delocalised structure is of higher energy than the localised structures [181]. 

We strongly advocate a return to Kekule position that aromaticity should be characterized by the structural features of a molecule. We not only should reject the use of properties for classification of aromaticity, but should continue to repudiate approaches based on properties of molecules for characterization of aromaticity as being inherently prone to fallacious conclusions. If properties are used as criteria for classification of compounds, they will continue to blur the boundaries between fully aromatic, aromatic, non-aromatic, and anti-aromatic molecules because properties may vary continually between molecules. In contrast, structural components are either present or absent in a molecule, and thus a molecule either qualifies or does not qualify to belong to a particular class. 

In all cases shown in Figure 90, we have only two Kekule valence structures. However, fully antiaromatic systems can have more than two Kekule valence structures, as illustrated in Figure 91. The expressions for their anti-aromatic destabilization energy are listed in Table 38. We see that the presence of only An conjugated circuits is a necessary condition for a structure to be a candidate for being anti-aromatic, or fully anti-aromatic . However, this is not sufficient. For such compounds to be truly antiaromatic, they must be planar and have all CC bonds of approximately equal length. 

At the top left in Figure 101 we illustrate one of the four Kekule valence structures for one of the isomers of pyracylene. We decomposed this valence structure into its conjugated circuits R2, Rz, and Q3, outlined in the center of the top row over the molecular skeleton as the peripheries of azulene (twice) and the periphery of the molecule as a whole. We assume in this qualitative representation that the two Rz conjugated circuits will contribute a current of strength +1 in the positive (anti-clockwise) sense to each bond of the azulene periphery. On the other hand, the bonds that belong to the molecular periph-... 

Figure 127. Fries and anti-Fries Kekule valence structures of Ceo.

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