Carbon networks

More important, tire surface curvature of tire carbon network exerts a profound impact on tire reactivity of tire fullerene core [6, 7]. In tliis context, tire most striking consequence emerges from tire pyramidalization of tire individual carbon atoms. Influenced by tire curvature, tire sp hybrids which exist in tmly two-dimensional planar... 

Similar to the fullerene ground state the singlet and triplet excited state properties of the carbon network are best discussed with respect to the tliree-dimensional symmetry. SurjDrisingly, the singlet excited state gives rise to a low emission fluorescence quantum yield of 1.0 x 10 [143]. Despite the highly constrained carbon network,... 

For the experiments referred to in Fig. 4.25(a), McEnaney was able to show, on reasonable assumptions, that the stress induced by adsorption swelling should be sufficient to fracture the carbon over short distances. A memory effect in the carbon network would lead to trapping of some adsorbed carbon tetrachloride molecules during the desorption run. 

The covalent compounds of graphite differ markedly from the crystal compounds. They are white or lightly colored electrical insulators, have Hi-defined formulas and occur in but one form, unlike the series typical of the crystal compounds. In the covalent compounds, the carbon network is deformed and the carbon atoms rearrange tetrahedraHy as in diamond. Often they are formed with explosive violence. 

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

The MejSi protecting groups in 58 a and 59 a could be removed by treatment with sodium tetraborate (borax) in MeOH/THF, yielding 58c and 59 c, respectively, as very unstable compounds [84]. Any attempts to obtain characterizable two-dimensional all-carbon network structures [3,4] by oxidative polymerization of 59 c have failed. 

The link between cyclo[ ]carbons and tetraethynylethene is the occurrence of both structural motifs as repeat units in fascinating two-dimensional all-carbon networks [3,4]. The development of viable preparative approaches toward these elusive acetylenic networks represents one of the true challenges for synthesis at the turn of the millennium. 

But not only organic stannylacetylenes coupled to 62 organometallic stanna-nes also gave rise to the formation of pentametallic complexes 64-67 in fair to good yields, and the cross-shaped complex 64 already is a sizable segment out of the proposed organometallic all-carbon network. It was possible to obtain X-ray... 

The TED and XRD patterns revealed that the deposit is not amorphous carbon but nanocrystalline diamond. Nonetheless, the 514-nm excited Raman spectra do not exhibit a clear diamond peak at 1332 cm though the peak due to the sp -bonded carbon network appears at 1150 cm The Raman cross section of the sp -bonded carbon network with visible excitation is resonantly enhanced [43, 48-50]. It consequently makes the 1332 cm diamond peak overlap with the peaks due to sp -bonded carbon. 

Radialene 52 has been envisaged as a precursor to hexaethynyl[3]radialene, a potential building block for carbon networks. However, desilylation under very mild conditions led to an unstable product of so far unknown identity. In this context, it should be mentioned that according to thermochemical calculations, the still unknown hexaethynyl[3]radialene has an increased conjugation energy with respect to vinylacetylene, probably because of partial relief of strain in the radialene corc26b. A remarkable aspect of 52 is its color the crystals are deep-red and a hexane solution has a purple color. In this respect, 52 differs from the yellow radialenes 50 and 51 and resembles [3]radialenes such as 27 and 38, which have much more extended --systems. 

Acetylenic Molecular Scaffoldings From Perethynylated Building Blocks to All-Carbon Networks and Carbon-Rich Nanostructures... 

Several two-dimensional all-carbon networks comprise the core of tetraethynylethene (TEE 17) as a monomeric repeat unit11,121 In 1991, we synthesized the hitherto dusive perethynylated ethene 17 on the way to these novel materials.1361 In the... 

The principle of building these planar carbon networks can also be extended to the third dimension in a manner similar to the construction of ful-lerenes from planar graphite sheets Thus, fullere-... 

U. H. F. Bunz, Y. Rubin, Y. Tobe, Polyethynylated Cydic re-Systems Scaffoldings for Novd Two- and Three-dimensional Carbon Networks , Chem. Soc Rev. 1999,28, 107-119. 

Schematic conversion of polyacrylonitrile by heating, into eventual polymer carbon network.

As for the covalent type, modification of CNTs by this approach has as its first goal to lead to debundling of the tubes, thus increasing their solubility and facilitating their manipulation. However, while the covalent method destroys the extended aromatic framework, noncovalent interactions preserve the original regular carbon network. This is important in those applications requiring use of the nanotubes without alteration of their electronic and optical properties, a process that normally occurs when the aromatic periodicity is disrupted. 

Recently, microscopic-level research has developed very small carbon networks called nanotubes. As you can see in Figure 4.19D, nanotubes are like a fullerene network that has been stretched into a cylinder shape. Nanotubes of C400 and higher may have applications in the manufacture of high-strength fibres. In the year 2000, researchers built a nanotube with a diameter of 4 x 10 m. Up to that time, this nanotube was the smallest structure assembled. 

A further aspect of this chemistry is in relation to the preparation of extended carbon networks and related systems. Imaginative consideration of various as yet... 

The bowl-shaped conj ugated hydrocarbons represent an attractive system for testing the theory of aromaticity since the curved surface of the carbon network represents a compromise between strain and conjugation. 

Due to its high symmetry, 8 exhibits only one signal in its H NMR spectrum (7.81 ppm), and three NMR lines at 127.2 ppm (10 methine rim carbons), 130.8 ppm (5 quaternary rihi carbon atoms), and 135.8 ppm (5 quaternary hub carbon atoms). The proton resonance of 8 is significantly shifted upfield, as compared to coronene 9 (8.9 ppm), which may reflect an influence of the curvature of the carbon network on the ring current. 

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