Polyacetylene isomers

Density functional theory was also applied to this problem. The cumulenic Dg, form IS turned out to be a minimum structure at the B-LYP level of DFT using a 6-31G basis set, while the fully symmetrical structure 13 is 2.4 kcal/mol higher in energy. Unfortunately, it was not possible to localize a polyacetylenic structure with the DFT methods used. The same trend holds for Cjo and Cj4, other candicates for Htickel aromaticity the and D-j symmetrical cumulenic structures are minima on the potential energy surface polyacetylenic isomers are not stationary points using DFT methods. 

Table 2 Computed equilibrium geometries of polyacetylene isomers l3 ...

The discovery of junipal focused the attention of Sorensen, who had been investigating the occurrence of polyacetylenes in Com-positae, on the possibility that these acetylenes were accompanied by thiophenes. From Coreopsis grandiflora Hogg ex sweet, 2-phenyl 5-(1-propynyl) thiophene (240) was isolated and its structure confirmed by synthesis of the tetrahydro compound, 2-phenyl-5-n-propyl-thiophene. From the root of tansy, the cis and trans isomers of methyl 5-(l-propynyl)-2-thienylacrylate (241) have been isolated. The total synthesis of trans (241) was achieved by reacting junipal with methylcarbethoxy triphenylphosphonium bromide (Wittig reaction) Several monosubstituted thiophenes, (242), (243), and... 

The acetylene reaction has been studied also at 77 K [409]. It has been reported that at this temperature the reaction occurs at 12.5 GPa and that the reaction proceeds to saturation and then accelerates on pressure release. In this low-temperature experiment, it has been found that the product contains also c -polyacetylene and that a transformation to the trans isomer occurs on heating. 

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

As in any other chemical compound, different geometrical arrangements of substituent groups are possible in a polymer where rigid molecular units are involved. This gives rise to trans- and cis-configurational isomerism in polymers containing double bonds in their repeat units, as in polyacetylene and natural and synthetic rubbers. The structures of the trans- and m-isomers of polyacetylene and polybutadiene are illustrated in Fig. 1.8. 

Polyacetylene aldehydes undergo a curious dimerization with loss of carbon monoxide " - . The reaction occurs spontaneously when concentrated solutions of the aldehydes 230 are allowed to stand at room temperature giving both Z and E isomers of the dimeric aldehydes 231. p-Substituted 5-phenyl-2,4-pcntadiynals (232)... 

THG measurements on cis- and rrani-polyacetylene were carried out over the pump frequency range from 0.55 eV though 1.25 eV [224,225]. The results show that x X3w (o,(o,a)) for rra i-polyacetylene is an order of magnitude larger than that for the cw-isomer over the entire spectral range, even comparing the respective 3o) resonance maxima. The results are summarized in Fig. VE-2 the solid points represent trans- CH and the open circles represent c/s -(CH),. The... 

The reaction of dienophiles with polyacetylene (33) was investigated for limiting the decomposition of the polyene. Previously, intermolecular Diels-Alder reactions involving ds bonds were believed to form cross-links and render polyacetylene intractable, and that their elimination would lead to soluble products (34). No reaction of maleic anhydride or benzoquinone with polyacetylene was observed. In contrast, a second study (35) described the successful formation of the Diels-Alder adduct of maleic anhydride with remnant cis bonds in thermally isomerized samples of fran -polyacetylene. These data were used to support the notion that the thermal isomerization process used to convert cw-polyacetylene to the trans isomer does not go to completion. Unfortunately, the effect of this transformation on the tract-ability or stability of the polymer was not noted. 

Both cis and trans forms of polyacetylene can be made with the trans isomer being the most stable. 

Polyacetylene is a colorful polymer. The cis isomer transmits red light, the trans isomer blue, and because these polymers often come in mixtures of cis and trans, various shades of purple result. If well formed films of polyacetylene are made, the surfaces reflect silver and, sometimes, gold. Polyacetylene in powder form appears black. The vacuum line experiments produced membranes which appeared purple when wet with solvent (transmitted light - the membrane is transparent when wet) and silver when dry (reflected light-the membrane is opaque when dry). 

Polyacetylene (CH)X is one of the simplest conjugated organic polymers. A number of quantum-chemical calculations with respect to the electronic structure of this substance have been accumulated up to the present. There can be distinguished two geometrical isomers of (CH) chains, namely, trans and cis. The trans- and cis-type chains are further classified into two and three structural isomers, respectively, in terms of the relative position of the C=C bonds. 

Typically, PA is synthesized as the m-isomer and the fran -isomer can be obtained from the cw-isomer by heating. The first NMR measurement on polyacetylene was carried out by Maricq et al. [2] by the CP/MAS technique. The chemical shift positions differ by 10 ppm the peak position of the frans-isomer is 139 ppm and the ds-isomer is 129 ppm. Gibson et al. studied the isomerization of the cis- to fran -polyacetylene by heating. As shown in Fig. 16.1, the sample without heating showed only one peak (128.5 ppm) which is characteristic of ds-polyacetylene. By isomerization at high temperature, a tram-peak appears at 137.3 ppm. It is weak at first but increases in intensity as the heating continues. Finally, the ds-peak disappears completely. Terao et al. [3] found that at the middle stage of isomerization, the tram-peak position shifts to low frequency with respect to that of a... 

The electronic structures of poiy(fluoroacetylene) and poly(difluoroacetylene) have been investigated previously using the ab initio Hartree-Fock crystal orbital method with a minimum basis set (42). Only the cis and trans isomers with assumed, planar geometries were studied. The trans isomer was calculated to be more stable in both cases, and the trans compounds were predicted to be better intrinsic semiconductors and more conductive upon reductive doping than trans polyacetylene. However, our results show that head-to-tail poly(fluoroacetylene) prefers the cis structure and that the trans structure for poly(difluoroacetylene) will not be stable. Thus the conclusions reached previously need to be re-evaluated based on our new structural information. Furthermore, as noted above, addition of electrons to these polymers may lead to structural deformations that could significantly change the conductive nature of the materials. 

An extensive comparative study on the performance of ab-initio and semiempirical methods when applied to carbon clusters C2 to Cjo was presented by Martin et al. [231]. For larger systems (n = 18 to 60), Feyereisen et al. performed SCF MP2 calculations in order to answer the question whether these molecules adopt closed fullerene or cumulene/polyacetylene ring structures. For s 32, the fullerenes were predicted to be the most stable isomers [232]. 

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