Cyclooctatetraene geometry

FIGURE 11 12 Molecular geometry of cyclooctatetraene The ring is not planar and the bond distances alternate between short double bonds and long single bonds... 

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... 

Problem 15.7 Cyclooctatetraene readily reacts with potassium metal to form the stable cycio-octatetraone dianion, C8H82. Why do you suppose this reaction occurs so easily What geometry do you expect for the cyclooctatetraene dianion ... 

SINDOl calculations, which successfully reproduced both the geometry and the aromaticity of the cyclooctatetraene dianion, predict a high degree of bond localization and nonplanarity for 1,4-dioxocin and its derivatives (84JOC4475). 

This geometry precludes the possibility of two equivalent VB structures, as for benzene, because, as you will see if you try to make a ball-and-stick model, 25b is highly strained and not energetically equivalent to 25a at all. Thus we can conclude that the delocalization energy of cyclooctatetraene is not large enough to overcome the angle strain that would develop if the molecule were to become planar and allow the tt electrons to form equivalent tt bonds between all of the pairs of adjacent carbons. 

Devise an atomic-orbital model for cyclooctatetraene in accord with the geometry expressed by formula 25a (Section 21-9A) and explain why electron delocalization is not likely to be important for a structure with this geometry... 

Examine the geometry of cyclooctatetraene dianion. Is it planar If not, describe its shape. Are all the carbon-carbon bond lengths the same If so, are they the same length as those in benzene If not, do they alternate between single and double bonds Do your observations suggest that cyclooctatetraene dianion is aromatic Is this in accord with Hiickel s rule ... 

Figure 18. (a) jt—a-Interplay diagrams for the twin states of /7-electron—/7-center antiaromatic species. (b,c) Calculated geometries for the twin states of cyclobutadiene and cyclooctatetraene (from refs 164a, 177, 188, and 219). 

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... 

Because of its nonplanar geometry, cyclooctatetraene is not antiaromatic and its hydrogens appear at 5.75 8, a value typical for alkenes. However, the triple bonds of the compound called benzo-l,5-cyclooctadiene-3,7-diyne force this molecule to assume a nearly planar geometry. The pi system of its eight-membered ring contains eight electrons. (Only two of the electrons of each triple bond are part of the conjugated system.)... 

Cyclooctatetraene readily reacts with potassium metal to form a dianion. Discuss the electronic structure and geometry of this dianion and explain why it is formed so readily. 

Spin-coupled calculations at the idealized Dm geometry of cyclooctatetraene reveal a description dominated by triplet coupling of pairs of electrons [12], as anticipated earlier. Expressing the total spin function in the Serber basis [29], we find that the mode made up only of triplet-coupled pairs is responsible for 75% of the total. We find that the n orbitals for this antiaromatic system (see Figure 9) adopt localized forms that resemble closely those shown in Figure 4 for benzene, rather than the antipair representation shown for cyclobutadiene in Figure 6. 

Figure 9. Symmetry-unique spin-coupled orbitals for cyclooctatetraene. Left idealized Du geometry (contours in the horizontal plane 1 bohr above the molecular plane). Right analogous representation for the nonplanar equilibrium geometry (DJ).

---