Polyacetylene ground state structure

Two recent ab-initio studies are devoted to the determination of the ground-state structure of Cg. Parasuk and AlmlOf concluded that the cyclic polyacetylenic structure ( Ag) and the linear cumulenic g state are essentially isoenergetic [260]. However, the choice of basis sets and methods (MRCI modified coupled-pair functional, MCPF) heavily affects the difference in energy between the linear and cyclic forms of Cg, and so the results are quite uncertain. On the other hand, Slanina et al. proposed a nonplanar Z)2d symmetrical cyclic structure as the minimum-energy structure of Cg [261]. According to their MP2/6-31G calculations, this form is 13 kcal/mol lower in energy than the planar Qa structure. Unexpectedly, all the C-C bond lengths in the species are equivalent (1.339 A). DFT calculations favor the linear structure as compared with a planar cyclic one, but nonplanar species seem to have not been considered in this study [236]. 

Contrary to the case of trons-polyacetylene with a degenerate ground-state structure, most other conju-... 

Solitons are neutral defects which arise as a consequence of the degenerate ground state structure of tmns-polyacetylene. J/ans-polyacetylene has a conjugated carbon backbone as shown in Figure 1. Each carbon is sp2 hybridized forming bonds with two adjacent carbons and one hydrogen. The pz orbitals lie perpendicular to the polymer chains and... 

Recently the theoretical models developed for polyacetylene have been applied to PDAs (00, 115, 116). A degenerate ground state does not occur for PDA so that this discussion is limited to polarons. The potential surface for the ground state structures deduced by this method is very similar to that calculated by the ab initio method (00, 106) indicating that the alternative butatriene structure is metastable. A further conclusion is that the band gap is primarily due to the bond alternation produced when the TT-electrons are added to the bare PDA backbone. These calculations are currently being extended to make realistic exciton calculations for PDAs (117). The occurrence of acousticmode polarons and exciton-polarons in PDAs has also been discussed recently (95, 145). 

In the case of polyacetylene the total energy of the systems as function of the dimerisation amplitude (fig. 1) shows two equal minima as the results of the Peierls distortion which favours dimerisation [2]. If we consider the case of polyaromatic polymers for which the ground electronic state is not accidentally doubly degenerate (fig. 2) the coordinate Jl can be defined as the collective geometrical evolution of the network of double/single CC bonds in transforming from the "aromatic" ground state structure to the excited electronic state described as "quinoid" (fig.2)... 

Polyacetylene is a polymer which has a degenerate ground state it possesses two geometric structures having exactly the same energy and differing only in the sequence of the carbon-carbon single and double bonds (Fig. 9.6). 

The other isomer is called cis-polyacetylene. This has a slightly different structure with non-degenerate ground state energy. 

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