Carbyne 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. 

It became obvious that a linear carbyne structure could not be accepted as a viable explanation for the observed stability of such a huge molecule. An... 

The unsubstituted ( naked ) C4 bridge offers an even larger electronic and structural flexibility when spanning two metal centres and several examples for redox-induced conversions between butadiynediyl, butatrienylidene and ethy-nylbis(carbyne) structures have been reported. Particularly instructive examples come from the work of Bruce et on C4-bridged diruthenium... 

Kavan, L. Kastner, J. Molecular and electron spectroscopy of carbyne structures. Raman and infrared spectroscopy. In Carbyne and Carbynoid Structures, R.B. Heimann, S.E. Evsyukov, L. Kavan, Eds., Physics and Chemistry of Materials with Low-Dimensional Structures, Kluwer Academic Publishers Dordrecht, Boston, London, 1999, Vol. 21, pp. 343-356. 

Yet another serious problem that makes the preparation of perfect carbyne structures (long-chain polyynes or cumulenes) an almost hopeless endeavor is the presence of intrinsic defects in the original halogen-containing polymers. The polymers produced by common polymerization technologies always contain some defect structures such as head-to-head and "tail-to-tail links [32-34] as well as branching sites [35,36]. For instance, poly(vinylidene fluoride) was shown to contain up to 6 mol.% [32,33] and poly(vinylidene chloride) even 12.5% of abnormal links [34], which hamper exhaustive dehydrohalogenation (Scheme 12.9). 

In summary, the assembly of benzene and carbyne molecules in circumstellar environments could potentially lead to formation of nanocrystals with the new carbyne structure (Figure 16.5). Still, a more likely scenario... 

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