Polyyne structure

The approach to form carbon ring molecules from organic precursors has been extended to produce three-dimensional macrocyclic polyynes of larger sizes (Section 6.4). Despite the fragility and explosive reactivity, the precursory molecules containing polyyne structures are successfully isolated and well characterized. The liberated pure carbon molecules have been... 

It was therefore concluded that the polyyne structure is not involved in the black insoluble particles. 

Therefore, it is evident that further elimination of HF should give rise to the formation of analogous head-to-head polyhalovinylene, and complete dehydrohalogenation must yield the polyyne structure (Scheme 12.5). 

Linear C species may be represented simply as shown in Fig. 1-6. For an even number of carbons, the simplest electronic structure may be either a dicarbene-cumulene structure, or a diradical-polyyne . The corresponding cyclic structures will be nonlinear and strained, but formally possess closed-shell cumulene or polyyne structures. These differ by having all bond lengths equal, or alternating bond lengths, respectively. For it = odd, the linear structures may be of the dicarbene-cumulene or tetraradical-polyyne type. The cyclic isomers may be cumulene or carbene-polyyne . 

With sp bond angles calculated to be around 162°, macrocycle 131 would be highly strained and was therefore expected to be quite reactive [79]. The octa-cobalt complex 132, on the other hand, should be readily isolable. Indeed, 132 was prepared easily from 133 in five steps, and was isolated as stable, deep maroon crystals (Scheme 30). All spectroscopic data supported formation of the strain-free dimeric structure. Unfortunately, all attempts to liberate 132 from the cobalt units led only to insoluble materials. Diederich et al. observed similar problems when trying to prepare the cyclocarbons [5c]. Whether the failure to prepare these two classes of macrocycles is due to the extreme reactivity of the distorted polyyne moiety or to the lack of a viable synthetic route is not certain. Thus, isolation and characterization of smaller bent hexatriyne- and octatetrayne-containing systems is an important goal that should help answer these questions. 

It should be emphasized that the electrochemical carbonization proceeds, in contrast to all other common carbonization reactions (pyrolysis), already at the room temperature. This fact elucidates various surprising physicochemical properties of electrochemical carbon, such as extreme chemical reactivity and adsorption capacity, time-dependent electronic conductivity and optical spectra, as well as its very peculiar structure which actually matches the structure of the starting fluorocarbon chain. The electrochemical carbon is, therefore, obtained primarily in the form of linear polymeric carbon chains (polycumulene, polyyne), generally termed carbyne. This can be schematically depicted by the reaction ... 

There are principally two pathways to encapsulation of polyyne chains in the voids of the porous host, viz. the intraporous polymerization of a suitable monomeric precursor and the intercalation of soluble oligoynes followed by their propagation. The former way includes the penetration of the porous structure with a soluble monomeric precursor and the subsequent intraporous polymerization, which can be started, e.g., by the UV irradiation. The latter one... 

FTIR spectra of the polymeric carbon formed in the channels of the MCM-41 by both the photochemical polymerization of C4I2 and the polymerization of Cmesoporous structure of the MCM-41 protects the reactive polyyne chains. Moreover, the confining of C6HI in the channels of the MCM-41 eliminates the explosive behaviour of the polymeric product. 

Despite the protective role of the MCM-41 porous structure, the stability of polyyne sequences in carbon chains against the heat exposition is limited. When all the materials prepared are heated in air at 100°C, the polyyne band in the IR spectrum diminishes. The heating at 160- 180 °C in vacuum of 10 1 Pa leads to the release of iodine and the disappearance of the polyyne band. 

Figure 2. Structure of Me3Si(C=C)4SiMe3 showing the curved polyyne chain arising from crystal packing forces (Coles et al. 1985). (Reprinted with permission of the Royal Society of Chemistry.)...

D. R. M. Walton. The early polyyne papers were by Jones and by Bohlmann, and to my knowledge, the crystal structures were not published, the latest papers are by Diederich and deal with cyclic carbon oxides. Don t press me for details over bond lengths, however there is a tendency towards bond equalization and the curvature is there. 

FIGURE 3. Chemical structures of platinum(II) polyyne polymers 1—75. 

FIGURE 15. A platinum(II) polyyne-based photocell in (a) single-layer and (b) bulk heterojunction structures. 

Carbynes are supposed to be formed of sp-hybridized carbon atoms bound linearly, where two n electrons have to be involved, giving two possibilities, i.e., an alternative repetition of single and triple bonds (polyyne) and a simple repetition of double bonds (cumulene) (Figure 2.1) [19]. The detailed structure of carbynes is not yet clarified, but some structural models have been proposed [19-22], A structural model is illustrated in Figure 2.11, where some numbers of sp-hybridized carbon atoms form chains that associate together by van der Waals interaction between jr-electron clouds to make layers, and then the layers are stacked. Foreign atoms are intercalated between the layers that are supposed to stabilize the carbyne structure. In the carbyne family, the variety of structures seems to be mainly due to the number of carbon atoms forming a linear chain, in other words, to the layer thickness, and to the density of chains in a layer. 

The polymer polyyne, -(C=C) , also known as polycarbene, and shown in Fig. 1.4(c), is formed with sp hybrids in this way. Its structure may be either an alternation of single and triple bonds or a sequence of double bonds, depending on the terminal groups - see Fig. 1.5. 

Fig. 1.5 The alternative structures of polymers formed with. -hybridised orbitals, (a) polyyne and (b) polycarbene. Overlap of the rc-electron orbitals, shown by the off-axis lines, gives an interatomic 7u-bond. The cr-bonds are shown by on-axis lines.

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