===Geometry===
In some PAHs, like naphthalene, anthracene, and coronene, all carbon and hydrogen atoms lie on the same plane.  This geometry is a consequence of the fact that the [[σ-bond]]s that result from the merger of sp<sup>2</sup> [[hybridization|hybrid]] [[orbital]]s of adjacent carbons lie on the same plane as the carbon atom. Those compounds are [[chirality|achiral]], since the plane of the molecule is a symmetry plane.  

However, some other PAHs are not planar.  Unlike the [[coronene]], corannulene adopts a bowl shape in order to reduce the bond stress.  Benzo[c]phenantrene (with four rings fused in "C" shape) is slightly distorted in a helical shape due to repulsion between the closest pair of hydrogen atoms in the two extremal rings.<ref name=herb1954>F. H. Herbstein  and  G. M. J. Schmidt (1954): "The structure of overcrowded aromatic compounds. Part III. The crystal structure of 3:4-benzophenanthrene". ''Journal of the Chemical Society'' (''Resumed''), volume 1954, issue 0, pages 3302-3313. {{doi|10.1039/JR9540003302}}</ref>. Adding another benzene ring to form dibenzo[c,g]phenantrene creates [[steric hindrance]] between the two extreme hydrogen atoms.<ref name=mike1976a>František Mikeš, Geraldine Boshart, and Emanuel Gil-Av (1976): "Resolution of optical isomers by high-performance liquid chromatography, using coated and bonded chiral charge-transfer complexing agents as stationary phases". ''Journal of Chromatography A'', volume 122, pages 205-221. {{doi|10.1016/S0021-9673(00)82245-1}}</ref>  Adding two more  rings on the same sense yields [[helicene|heptahelicene]] in which the two extreme rings overlap.<ref name=mike1976b>František Mikeš, Geraldine Boshart, and Emanuel Gil-Av (1976): "Helicenes. Resolution on chiral charge-transfer complexing agents using high performance liquid chromatography". ''Journal of the Chemical Society, Chemical Communications'', volume 1976, issue 3, pages 99-100. {{doi|10.1039/C39760000099}}</ref>  These non-planar forms are chiral, and their [[enantiomer]]s can be isolated.<ref name=gutm2012/>