Polyyne chains

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

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. 

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... 

The present study aims at the development of procedures for the generation of polyyne chains in the mesopores of the siliceous MCM-41 and the characterization of materials obtained. As precursors, l,4-diiodo-l,3-butadiyne (C4I2) and l-iodo-l,3,5-hexatriyne (CaHl), were used. 

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]. 

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.)...

Coupling constants for C- H have been measured, and are found to be very useful in making assignments -Scheme 1, the coupling of alkyl and acetylenic hydrogens with the carbons of a polyyne chain decreases regularly with... 

It was widely accepted that polyyne chains can be stabilized in structures where a large interchain distance is maintained by interdispersing foreign materials [7], Moreover, polycumulene structures have been predicted to be so fragile that the synthesis of a material containing this type of sp chains has be considered extremely difficult if not impossible [5]. 

According to Heimann et al. [48], for a polyyne chain, the length of carbon chain is given by... 

In CCI4 and CI ICI3 the high formation rate of pyrocarbon is accompanied by the complete absence of polyyne formation in the respective solutions checked by the HPLC-DAD. No PAHs were found only a mixture of halogenated derivatives. Evidently, the large amount of Cl radicals produced by the plasmalysis of these solvents is sufficient to easily chlorinate the polyyne chains or to cause their cyclization into other products. The results of these experiments are discussed in more detail in Reference 48. 

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... 

The temperature dependence of the d.c. electric conductivity of the sp films in the direction parallel to the chain axis shows that it is characterized by the thermally activated behavior and the activation energy is equal to 0.02 eV in the temperature interval 160-300 K. This is in good agreement with the activation energy of solitons in polyyne chains. 

It is worth reporting here that arcing at —40°C led us to also detect the hydrogen-capped polyynes having 18 carbon atoms, the longest polyyne chain we have detected so far [18,19] H-(C=C)9-H. 

The polyyne C18H2 was not detected in the acetonitrile solution arced at room temperature and was not found also in the methanol or hexane solutions arced at room and at low temperature. In these other cases the longest polyyne chain had only 16 carbon atoms. 

Since the end groups of polyyne chains are affected by the nature of the solvent where the graphite arc is struck, and since we know that usually the polyynes are hydrogen terminated when the arc is made in hydrocarbon solvents like n-hexane or n-octane while they are also cyano terminated when the arc is made in acetonitrile, these spectral differences can be taken as a first hint that the polyynes formed in liquid nitrogen are nitrogen and hence cyano terminated. 

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