Polycyclic aromatic compounds nonplanarity

The realization that polycyclic aromatic compounds are not necessarily planar is not a new concept. A book published in 1964 on benzenoid hydrocarbons (24) contains a chapter on nonplanar, overcrowded, aromatic hydrocarbons this chapter primarily describes high degrees of nonplanarity that are due to steric interactions of hydrogen atoms in benzenoid compounds that contain substructures related to benzo[c]phenanthrene (5) (twist angle = 31°) or to dibenzo[c,d]phenanthrene (6) (twist angle = ca. 50°) (26-28). 

Structures determined by modern X-ray techniques show that significant nonplanarities exist in benzenoid compounds of this type. This finding infers that nonplanar polycyclic aromatic compounds are quite common. Structures 7-9, which have twist angles ranging from 30° to 42°, are examples... 

AU of the PAH and super acenes are planar molecules. However, incorporation of a seven-membered ling results in nonplanar (curved) molecules that are of interest in the context of curved % surfaces and nonplanar aromatic compounds. Cycloaddition/dehydrogenation strategy shown in Scheme 16.42 results in the formation of seven-membered ring fused curved polycyclic aromatic molecule [46]. 

Condensed polycyclic benzenoid aromatic hydrocarbons are customarily regarded as planar molecular structures because of the geometrical constraints of carbon atoms in a state of sp2 hybridization. A well-known exception is the class of compounds called the helicenes (18) for which the nonbonded overlap of two terminal benzenoid rings in a cata-condensed structure, as in structure 1, forces a molecule into a nonplanar helical structure. A second exceptional class of compounds is related to corannulene (2) and other an-nulenes of this type (19, 20). In corannulene, strain associated with the pericondensed five- and six-membered rings requires adoption of a bowlshaped structure (20, 21). For both structures 1 and 2 the aromatic character of the benzenoid rings is retained to an appreciable extent. 

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