I know [8]annulene is non-planar, but are all bond lengths equal?
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Background:
8-Annulene actually is supposed to be anti-aromatic but it twists to become non-aromatic. It has a tub-shaped structure.
Answer:
Given there are both single and double bonds, the bond distances must be of 2 kinds that is $\ce{C-C}$ and $\ce{C=C}$ distances.
It is also experimentally verified. X-ray diffraction information from H. S. Kaufman illustrated cyclooctatetraene to embrace a few conformations and to contain two different C–C bond separations. This result demonstrated that cyclooctatetraene is an annulene with settled rotating single and double C-C bonds.
This article states:
We have been engaged in an investigation of the structure of cyclo-octatetraene by X-ray methods. In a previous communication, we gave the preliminary crystallographic data. Cyclo-octatetraene crystallizes in the orthorhombic system with four molecules per unit cell. The cell dimensions are $\pu{a= 7.76 Å, b = 7.80 Å, c = 10.66 Å}$. The space group is Aba.
Four structures can be postulated for the cyclooctatetraene molecule (see Fig. I): the 'crown' form -all trans configuration ; the 'tub' form-all cis ; the 'chair' form-half cis, half trans; and the 'plane' form, which presumes a large resonance energy to overcome the strain. Raman and infra-red data obtained by Lord and Lippincott led these investigators to favour the crown form. Similar experiments by Thompsons indicate the tub form to be most probable. Based on their recent electron diffraction study, Hassel and co-workers conclude that the structure is the crown form with bond-lengths approximately those of benzene, suggesting a high degree of resonance. Bragg-Lipson structure factor charts proved very useful in our work. It was found that only the tub form with alternating single- and double-bonds gave a good fit with the experimental data (Fig. 2).
The two-fold axis required by the space group must pass through the single bonds, and the bond-lengths are typically aliphatic: double-bond length, $\pu{1.34 Å}$; single-bond length, $\pu{1.54 Å}$; and $\ce{C=C-C}$ bond angle $\pu{125°}$. These values indicate little or no resonance energy in the cyclo-octatetraene molecule, which is in accord with the latest thermochemical data of Rossini and co-workers.
References:
- University of Calgary - Chapter 11: Other Hydrocarbons
- Kaufman, H.S., Fankuchen, I., & Mark, H. Nature; 161; 1948; 165.
