Aromaticity, aromatic character nonaromatic systems

The replacement of some thiophene rings by other heteroaromatic rings, such as silole [49,50] or phosp-hole [51-53], represents an alternative solution to modulate the bandgap of PT. Thus, polymer 11 shows a relatively low value of 1.60 eV due to the low aromatic character of phosphole. Additionally, optical and electrochemical properties depend on the substitution at the phosphorus atom [51-53]. Similarly, insertion of the nonaromatic thiophene-S,S-dioxide in conjugated systems, such as 12, has been shown to produce an extension of the ir-electron delocali2ation associated with the reduction of bandgap [54—57]. 

All the peaks are somewhat upfield of the aromatic region, suggesting polyene character. This stmcture would also be consistent with the observed reactivity since the polyene has a. quinodimethane structure (see Section 11.3). The implication of a nonaromatic stmcture is that the combination of ring strain and the antiaromaticity associated with the four--nembered ring results in a localized system. ... 

The primary objective of most applications carried out so far was to assess the performance of the PPP-VB method for diverse alternant and nonaltemant -electron systems of aromatic, nonaromatic or antiaromatic character, both electrically neutral and charged. The main emphasis was on ground states of different spin multiplicity, even though some preliminary calculations were also carried out for excited states. The PPP-VB codes were also employed to provide the approximate three- and four-body connected cluster components for the so-called VB-corrected coupled cluster (CC) approach [71]. In the following, we briefly point out the most important aspects of the PPP-VB method and illustrate them with a few typical results. 

In this case, there are eight % electrons in eight p orbitals that are contiguous. The system shows nonaromatic character, e g. it is not flat and the single and double bonds are distinguished by being different lengths. If it were planar, it would be anti-aromatic, and this is one of the reasons why it is not planar. 

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