Polyynes linear

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. 3. A, A kinked polyyne chain model for linear carbynes (after [23]) B, eyclo C-18 earbync [25].

Trans-polyenes H-(-HC=CH-),, -H, trans-polyenynes H-(HC=CH-C=C) -H, cumulenes H2C=(C=C) =CH2 and polyynes H-(C=C) -H have been studied (M=N-1). For eentrosymmetrie molecules, the first order hyperpolarizability p is equal to zero so that non linear effects are of second order nature. Furthermore, (the x axis goes through the middle of the C-C bonds of the polyenes, or is the intemuclear axis in the case of linear molecules) is the most important component of the second order y hyperpolarizability tensor, the other components being negligible. Both y and the mean hyperpolarizability... 

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 ... 

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. 

Photoelectron spectra have been measured for other, more complex polyynes -The values obtained for a large number of polyynes can be correlated in a simple fashion in terms of a model based on linear combination of bonding orbitals -... 

In 1961, Wotiz and Dale independently reported the new syntheses of non-conjugated cyclic polyynes. When a,a -dibromides are treated with a mixture of sodium acetylide and disodium acetylide in liquid ammonia, cyclic diynes (129) are obtained together with the linear polyynes (130). Sodium acetylide, being a chain terminator, prevents the formation of large amounts of linear polyynes. 1,8-Cyclo-tetradecadiyne (129, n = 5) and the 22-membered dioxadiyne (131) were obtained by this method. 

Solids based on sp hybridization, although subject of intense experimental efforts, seem to be the most elusive of the different carbon families [5]. The existence of linear chains of carbon atoms linked by alternating single and triple bonds (polyyne) or double bonds (polycumulene) with stabilizing molecular complexes at the end of the chains, has been recognized in interstellar molecular clouds and can be artificially produced by different chemical routes [6-8],... 

In situ Raman spectroscopy on the films has confirmed the presence of polyynes and polycumulenes in a nanostructured sp matrix. These sp hybridized linear carbon structures are metastable even in UHV at room temperature. When exposed to oxygen, the carbon network structure rapidly evolves towards a more common mainly sp amorphous phase, with only a small residual amount of sp linear aggregates [23]. 

Our experiments show, unambigously, that a pure carbon film rich with sp linear chains can be formed by assembling carbon clusters at room temperature under ultra-high vacuum. Polyyne and polycumulene species are present and stabilized in all-carbon metastable structures without the need of hetero-atom terminal groups. Under He, N2, and H2 exposure the amount of sp species reduces exponentially with time depending upon the mass of the gas. For the investigated gases no chemical interactions have been observed. 

Poly(tetrafluoroethylene) (PTFE) is a promising precursor for carbyne. The linear polymeric chain of PTFE could be transferred into polyyne according to the ideal scheme ... 

Increasing interest has been focused on the acetylene-based polymer chemistry relevant to cyclic polyynes — the topic of this chapter [1]. Cyclic polyynes are unique members in the family of sp-carbon allotropes. Like linear polyynes, they are composed of unsaturated carbon atoms covalently... 

The kinetics of polyynes production with the submerged carbon arc appears linear with the arcing time and the amount of current or energy (power) supplied to the reaction system. 

The stability of Cio suggests a monocyclic form because linear Cio, whether cumulenic or polyynic, must be subjected to the attack by hydrogen atoms according to the discussion stated above. More convincing evidence for the cyclic form is desired, however. 

Another interesting examination of the crystallographically characterized oligoynes was the carbon chain conformations. Polyyne and oligoynes must be regarded as essentially linear , but it is suggested that types of carbon chain conformations are classified into (A) linear, (B) symmetric bow, (C) unsymmetrical bow, (D) kinked, (E) S-shaped, and (F) random from the crystallographical analyses. There was no example classified into strictly linear (A), while bow-shaped and S-shaped conformations (B and E) were quite common. 

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