How does the molecule consisting of two bridged aromatic rings look in 3D?

Question

I can't understand how the following compound with bridged benzene rings looks like:

Answer 1

This is a cyclophane, namely 4,8-dibromo[2.2]paracyclophane. Lonsdale et al. [1] first structurally characterized [2.2]paracyclophane in 1960:

The molecule consists of two facing benzene rings linked in para-positions by two $\ce{(CH2)2}$ bridges. As a result of steric hindrance, the benzene rings are each bent into a boat-shape, but even so, their maximum distance apart in the molecule is only 3.09 Å, whereas the normal van der Waals distance is upwards of 3.4 Å.

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The angle of bending of the benzene ring (14°) does not change with temperature, but the aliphatic $\ce{CH2-CH2}$ bond appears to be unusually long at room temperatures* and has decreased by 0.07 Å at 93 °K. Moreover, although the distance between the two benzene rings does not change, there is a decrease of 0.03 Å in the overall length of the molecule from 291 to 93 °K.

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Figure 4. Diagram showing projection of molecules at $0, 0, 0;$ $1/2, 1/2, 1/2$ and $0, 0, 1$ on to $(100).$

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* The $\ce{CH2-CH2}$ bond length is 1.63 Å, which is above average 1.54 Å expected for the aliphatic $\ce{C-C}$ bond.

See CCDC entry DXYLEN for more details and an interactive 3D image.

Several related polysubstituted [2.2]paracyclophanes have been reported:

• 5,7-dibromo-4-hydroxy[2.2]paracyclophane [2] (CCDC: FULNUX);
• 4,8,12,16-tetrabromo[2.2]paracyclophane [3] (CCDC: SAWDIE);
• 4,5,7,8,12,13,15,16-octabromo[2.2]paracyclophane [3] (CCDC: SAWDEA).

References

1. Lonsdale, K. Y.; Milledge, H. J.; Rao, K. K. V. Studies of the structure, thermal expansion and molecular vibrations of di-p-xylylene, $\ce{C16H16},$ at 93 and 291 °K. Proc. R. Soc. Lond. A 1960, 255 (1280), 82–100. DOI: 10.1098/rspa.1960.0055.
2. Atman Vorontsova, N. V.; Zhuravsky, R. P.; Antonov, D. Yu.; Sergeeva, E. V.; Godovikov, I. A.; Starikova, Z. A.; Vologzhanina, A. V. Synthesis of Bromo-Substituted 4-Hydroxy[2.2]Paracyclophanes and [2.2]Paracyclophane-4,7-Quinones as Versatile Chiral Building Blocks: [2.2]Paracyclophane Derivatives as Versatile Chiral Building Blocks. Eur. J. Org. Chem. 2015(2), 325–330. DOI: 10.1002/ejoc.201403316.
3. Chow, H.-F.; Low, K.-H.; Wong, K. Y. An Improved Method for the Regiospecific Synthesis of Polysubstituted [2.2]Paracyclophanes. Synlett 2005, No. 14, 2130–2134. DOI: 10.1055/s-2005-872270.