page_title: Contorted aromatics
section_title: Introduction
hierarchical_section_title: Contorted aromatics / Introduction
caption_attribution_description: English: Figure 2. 3D molecular diagrams showing arching Five-13,16,17,82,88,89,90 and splitting Six-7,16,36 interactions. Five: PAH containing five membered rings and Six; PAH containing only six membered rings2. The red arrows point to the bay regions.
context_page_description: Contorted aromatics or more precisely contorted polycyclic aromatic hydrocarbons are polycyclic aromatic hydrocarbons in which the fused aromatic molecules deviate from the usual planarity.
context_section_description: The comparison of structures of graphene and fullerene can help comprehend the origin of curved pi surfaces and contortion in PAHs.  Both graphene and fullerene bear resemblance in having sp² hybridized carbons however, exhibit different geometries (allotropes of carbon). The fact that fullerenes have five membered rings incorporated among six membered rings, makes them spherical whereas graphene remains planar due to the presence of exclusively all six membered rings in it. In general, the ideal C-C bond length and angle for C-C-C or C-C-H is about 1.42 Å and 2π/3 respectively whereas in corannulene structure, a five membered ring is surrounded by six membered rings contributing to non-planarity in the molecule. This leads to change in bond angles and bond lengths rendering non-planarity in the structure. Such type of  contortions in the structure of PAHs are known as ‘arching distortions’. The hydrogens and carbon atoms of the PAH molecule which are either closest to one another due to non-planar structure or suffer from angle strain are known as the "saturated" ones and may serve as a source of ‘splitting distortions’ Figure 2.
  The region of a contorted molecule having saturated hydrogens and carbons is known as bay region (having white spheres or saturated hydrogens) Figure 4.
  Another reason for making the PAHs molecules contorted may be the size of these molecules. Theoretical vibrational frequency studies of C₆n₂ H₆n (n=2-12) on coronenes using quantum chemical calculation (Hartee-Fock and DFT) show accelerated loss of planarity on increasing size of PAH molecules in-spite of having a stable structure due to  conjugation, delocalization and aromatization. A gas phase coronene can be expected to switch to non-planar geometry around n=9-12. The combined strain of arching and splitting distortion is thought to force the PAH molecules out of the plane and generate contortions.  A switch to umbrella geometry may occur at n=12 Figure 3. 
  Figure 4 illustrates the strain energies carried by three different PAH molecules. The white spheres represent saturated hydrogens and the grey spheres as the leading carbons contributing to strain energy due to non-planar distortions. 
  Table1 shows the energy values for non-planarity (strain) of PAHs in k cal/mole for five membered and six membered ring molecules with common helicene and coronene units with reference to fullerenes C₆₀=483.91 k cal/mol and C₇₀=492.58 kcal/mol. The shaded parts indicate the bay regions in PAH molecules bearing the most strain. The strain energy is expressed in units of 10-2  kcal/mol.
  The Enp non-planar strain (contortion) energies indicate how much a PAH molecule deviate from the standard planar structure in terms of bond angles and bond lengths. The standard for PAHs is usually graphite. An Enp is introduced in the molecular structure whenever it deviates from the standard structural geometries to get the strain relief. The Enp values represented for a variety of PAHs in Table 1 are shown. Two types of contortions or Enp can be observed based on the data in Table 1. The PAH molecules shown in table 1 are based on two kinds of motifs. The ones based on helicenes and the others on corannulenes. The small Enp values (ranging from 0.25-8 k cal/mol) represented by the helicene based molecules and are because of weak contortions in their structures. The contributing factor seems to be the presence of only splitting distortions in the bay region of these PAHs. However, the other group based on corannulenes show higher Enp values (ranging from 1.86-116 k cal/mol). The higher Enp values suggest stronger contortions which may have originated due to the higher number of fused rings and a five membered ring core in PAH molecules along with the bay region contortions.
  Based on the above discussion it can be cautiously inferred that the combined effects of non-planar strain caused by arching and splitting distortions as well as the size of PAH molecules give birth to contortions in PAHs structures.
  These unparalleled contorted molecules exhibit wide absorption spectr
caption: Fig. 2. 3D molecular diagrams showing arching Five-13,16,17,82,88,89,90 and splitting Six-7,16,36 interactions. Five: PAH containing five membered rings and Six; PAH containing only six membered rings2.
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