Conjugated polyenes linear, electronic structure

Kohler BE (1990) A simple model for linearpolyene electronic structure. J Chem Phys 93 5838-5842 Kohler BE (1991) Electronic properties of linear polyenes. In Bredas JLand Silbey R(eds) Conjugated Polymers The novel science and technology of conducting and nonlinear optically active materials, pp 405-434. Kulver Press, Dordrecht Kohler BE(1993a) Octatetraene photoisomerization. ChemRev 93 41-54... 

Butadiene. - The ab initio calculation of the linear polarizability and second hyperpolarizability tensors for f/ww-butadiene has attracted a great deal of attention. The molecule is the smallest stable structure showing -electron delocalisation and is the first member of the fully conjugated polyene series. Extended conjugated structures are associated with low excitation energies and large response functions. 

The carotenoids generally found in foods are linear zll-trans E form) polyenes formed from eight isoprenoid units. The structures of common carotenoids are shown in Figure 8.11. The linear conjugated polyene structure has the ability to delocalize an unpaired electron and hence the capacity to act to terminate free radical reactions with the production of resonance stabilized free radical structures. Thus, carotenoids may potentially (a) provide retinol and (b) act as antioxidants. 

By the 1960 s it appeared that the electronic structure of the linear polyenes, arguably the simplest conjugated organic molecules, was well understood. In 1972 we reported the... 

So, a polyene is simply a long chain of alternating single and double bonds a conjugated system. In our example, there are 16 rr-electrons, but the box length is rather undefined. Why Resonance structures exist and the 7r system is not really a linear chain. 

---