Poly benzenoid structure

In the majority of organic molecules covalent chemical bonds exist between distinct pairs of atoms. The bonding in such a molecule is then described by classical structural formula. There are, however, quite a few examples of molecules in which several (more than two) atoms are covalently bound by means of a chemical bond, so-called poly centric bond. Organometallic compounds and benzenoid hydrocarbons are examples of such kind of molecules. 

CH3 p-quinoid structure p-benzenoid biradical poly(p-xyiyiene)... 

Examination of the structure shows that the soliton defect cannot be supported in poly(p-phenylene), as there is no degenerate ground state. Instead, the two neariy equivalent structures are the benzenoid and quininoid forms, which have diffeimt mragies. 

The reader is referred to the Number of Benzenoids and Poly hex Hydrocarbons subsection for the definition and classification of benzenoids and polyhex hydrocarbons, as well as for additional references for this class of compounds, which is only partially reviewed because of space limitations. Let us recall that the direct counting approach has difficulties with molecules that cannot be represented by tree-like structures, such as pericondensed polyhexes. Furthermore, the counting approach caimot separate nonplanar polyhexes (helicenes) from planar benzenoids. Consequently, for benzenoids and polyhexes, enumeration is not only a valuable tool that provides a concise description of the structures being enumerated, but also enumeration computes isomer numbers that cannot be derived otherwise. 

Fig. 2-7 depicts some examples of actual structures that polarons and bipolarons represent in CP chains, illustrated for poly(p-phenylene) and poly (pyrrole). More such structures are presented further below. It can be seen that the region of the polaron/bipolaron in general represents a domain in which the normal, aromatic structure of an uncharged CP is interrupted in a significant way For many cases, this aromatic-ring, benzenoid (or benzoid) type structure of the CP is replaced in this region by a quinonoid (or quinoid) type of structure, as seen quite clearly in the figures. 

Fig. 2-8 Left Benzenoid/Quinonoid structures for Poly(p-phenylene), together with energy diagram. Right For Poly(isothianaphtheneX...

Draw structures showing benzenoid and quinonoid segments alternating for four monomer units for each of the following CPs poly(3-hexyl-thiophene) poly(3,3 -dimethyl pyrrole), poly(isothianaphthene) (a thiophene with a fused benzene ring). 

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