Molecular orbitals of cyclooctatetraene

Fig. 2. Correlation of the molecular orbitals of cyclooctatetraene with those of pentalene...

Energies of the pi molecular orbitals of cyclooctatetraene and the cycloheptatrienyl carbocation. Note that the electrons are not yet shown in the MOs in these diagrams. 

Figure 16-8 shows that the first 3 pairs of electrons are in three bonding molecular orbitals of cyclooctatetraene. Electrons 7 and 8, however, are located in two different nonbonding orbitals. As in cyclobutadiene, a planar cyclooctatetraene is predicted to be a diradical, a particularly unstable electron configuration. 

FIGURE 11 13 Frost s circle and the TT molecular orbitals of (a) square cyclobutadiene (b) ben zene and (c) planar cyclooctatetraene... 

Use the inscribed polygon method to show the pattern of molecular orbitals in cyclooctatetraene. 

Figure 4.26 shows the FfMOs for cyclopropenyl carbocation, square planar cyclobutadiene, cyclopentadienyl anion, benzene, cycloheptatrienyl carbocation, and planar (Ds/,) cyclooctatetraene. If we place electrons into the molecular orbitals of each species according to the aufbau principle, we notice an important relationship between the stability of the systems and the... 

FIGURE 14.7 The tt molecular orbitals that cyclooctatetraene would have if it were planar, Notice that, unlike benzene, this molecule is predicted to have two nonbonding orbitals, and because it has eight tt electrons, it would have an unpaired electron in each of the two nonbonding orbitals (Hund s rule. Section 1.11). Such a system would not be expected to be aromatic. 

FIGURE 13.22 The molecular orbitals of planar cyclooctatetraene. Because there are eight 7t electrons, two must occupy nonbonding orbitals. 

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). 

Figure 2.28 Relative energies of benzene and cyclooctatetraene molecular orbitals from Frost circles...

The simple resonance theory fails to explain the singular lack of effectiveness of delocalization in cyclobutadiene and cyclooctatetraene, but we may turn to molecular orbitals for the solution. 

Another aspect of the chemistry of M(CO) fragments that the computed molecular orbital diagram could help to explain was the structure of the cyclooctatetraene complexes of iron and chromium tricarbonyl.10 Chromium tricarbonyl was shown to have three relatively low-lying vacant orbitals, with the right spatial characteristics to be able to accept electron donation from three of the double bonds of cyclooctatetraene, and accordingly adopts the r geometry shown in Figure 10.4. In iron... 

Of course, this isn t the molecular orbital energy level diagram for real cyclooctatetraene since COT is not planar but tub-shaped. 

Although Hiickel molecular orbital theory is not completely consistent in its application to nonalternant systems such as the pentalenyl dianion, it does implicate a certain degree of stabilization for this species. If the transannular bond introduces little or no perturbation, then the pentalenyl dianion is seen to be closely related to the cyclooctatetraene dianion. Katz and his co-workers successfully developed a... 

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