Polyyne electronic structure

In this section we report our experimental findings relatively to three different reactions of CN radicals with simple alkynes, namely acetylene, methyl-acetylene and dimethyl-acetylene. We have selected these reactive systems for different reasons the reactions with C2H2 is the prototype for the class of reactions CN +- alkynes/polyynes, thus is expected to reveal key concepts for reactions with the higher members of the same series the reactions with methylacetylene and dimethylacetylene were selected to observe the effect of the H substitution with one or two alkyl groups. In all cases, the experimental results are discussed in the light of the ab initio electronic structure calculations for the stationary points of the relevant potential energy surfaces. 

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 . 

Figure 1-6 Electronic structures of odd and even length (n) linear and cyclic polyynes.

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. 

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.

In this chapter, keeping in mind the generation, characterization, and reactions of the cyclic polyynes, the interplay of organic chemistry and carbon cluster science during the last decade is presented. First, following short historical remarks (Section 6.2.1), recent research activity on the production of cyclo[ ]carbons from well-defined organic precursors is surveyed (Section 6.2.2). Second, major structural and electronic properties of mono-cyclic carbon clusters are presented in the context of theoretical considerations (Section 6.2.3), followed by observational results of photoelectron spectroscopy (Section 6.2.4). Third, considerations on the infrared activity of cyclic Cio will be presented (Section 6.2.5). Finally, this chapter ends with experimental as well as theoretical proposals for the structures of multicyclic polyynes (Section 6.3) and their relevance to the formation of fullerenes, in particular from polycyclic polyynes (Section 6.4). 

In this chapter, I have attempted to give an overview of the field of polyyne-type materials. End-capped oligoynes supply fundamental properties about polyyne chains e.g. chemical reactivity, electronic properties, structural information, and so on. Certain characteristics of each oligoyne lead to... 

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