Polyyne carbon

Carbynes are a form of carbon with chains of carbon atoms formed from conjugated C(sp )=C(sp ) bonds (polyynes) ... 

X-ray diffraction peaks were rather broad with coherence lengths as low as 20 nm and this was attributed to rapid quenching. It was proposed that the carbon atoms are arranged in polyyne chains (n = 4) along the c-axis. The density of Carbolite (1.46 g-cm ) is lower than values for other carbynes and for diamond and graphite - hence the name - and this was attributed to a rapid quenching process. 

In addition to questions over interpretation of diffraction data, diere are reservations about the stability of carbynes. Lagow et al [32] note that the condensation of the compound Li-C=C-Br to form carbon chains is potentially explosive. There is also the possibility of cross-linking between carbyne chains and the nature of the termination of the carbyne chains is unclear. Eastmond et al [33] showed that polyyne compounds of the type ... 

Much current research is centering on polyynes—linear carbon chains of sp-hybridized carbon atoms. Polyynes with up to eight triple bonds have been detected in interstellar space, and evidence has been presented for the existence of carbyne, an allotrope of carbon consisting of repeating triple bonds in long chains of indefinite length. 

Fig. 1. Cyclo[n]carbons in their polyyne and their cumulenic forms...

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 ... 

Compounds of this type, listed in Table 9, are available by interaction of triallylboranes with certain polyyne derivatives of silicon and tin. Their formation involves 1,1- and 1,2-allylboration of triple and double carbon-carbon bonds. 

Having initiated the chemistry within the atmospheric model, the elongation of the carbon-chain polyyne species occurs by successive addition of the C2H radical ... 

After explosive decomposition of this polymer, a polymeric carbon is formed, which does not display any polyyne bands in the FTIR spectrum. The iodine, which was released by the decomposition of the polymer, could be added to C6HI yielding finally the dimer C 2H2Ig ... 

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. 

The elemental composition of MCM-41 samples with encapsulated polymeric carbon is presented in Tab. 1. All these materials contain relatively large amounts of iodine, which probably improves the stability of polyyne chains [14]. 

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. 

In conclusion we stress again that polyynes are central to carbon clusters and that this area of endeavour continues to represent a challenge when it conies to interpreting complex chemistry as epitomized by the fullerenes. 

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. 

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. 

It has become clear that the carbon-rich hb-PYs are readily curable (from ca. 150 °C), thermally stable (up to ca. 550 °C), and pyrolytically carboniz-able (yield up to 80% at 900 °C). Furthermore, their triple bonds are easily metallizable by the complexations with cobalt carbonyls. Since the polymer complexes contain a large number of metal atoms, we tried to utilize them as precursors for fabrication of metalloceramics. The pyrolyses of the polyyne-cobalt complexes at 1000 °C for 1 h under nitrogen furnished ceramic products 83 and 84 in 50%-65% yields (cf., Scheme 31). All the ceramics were magnetizable and could be readily attracted to a bar magnet. 

---