Carbon clusters production

A much more detailed and time-dependent study of complex hydrocarbon and carbon cluster formation has been prepared by Bettens and Herbst,83 84 who considered the detailed growth of unsaturated hydrocarbons and clusters via ion-molecule and neutral-neutral processes under the conditions of both dense and diffuse interstellar clouds. In order to include molecules up to 64 carbon atoms in size, these authors increased the size of their gas-phase model to include approximately 10,000reactions. The products of many of the unstudied reactions have been estimated via simplified statistical (RRKM) calculations coupled with ab initio and semiempirical energy calculations. The simplified RRKM approach posits a transition state between complex and products even when no obvious potential barrier... 

A unique example is the production of a polymer that contains Cgo- The fuller-ene monomer was prepared by cycloaddition of quadricyclane followed by polymerization in norbomene at room temperature.235 The product has cis trans ratios between 3 1 and 6 1 depending on the concentration of the monomer. A processable film containing 1% C60 exhibits electronic and electrochemical properties that are typical of the carbon cluster. 

An alternative starting material for carbon fiber production is pitch—a complex mixture of fused polyaromatic hydrocarbon clusters that can also be melt-spun into fibers. 

O Keefe, A. O., Ross, M. M. Baronavski, A. P. 1986 Production of large carbon cluster ions by laser vaporisation. Chem. Phys. Lett. 130, 18-19. 

An argument against mechanisms utilizing particular intermediate size clusters can be based on the fact that a wide variety of intermediate size clusters are observed in carbon cluster distributions which ultimately will produce larger clusters. Thus it is hard to account for the high yields of Cff when these other intermediates pathways, which presumably lead to other products, are present. 

Rohlfing, E. A., Cox, D. M. Kaldor, A. 1984 Production and characterisation of supersonic carbon cluster beams. J. chem. Phys. 81, 3322-3330. 

It may be safely suggested that origin of the ferromagnetic state in the synthesis products is a result of change in their electronic structure at the high-energy plasmochemistry synthesis. It results in the appearance of carbonic clusters with unpaired electron spins and formation of ferromagnetic domains with equally oriented spins. 

Bigger clusters have been formed, for instance, by the expansion of laser evaporated material in a gas still under vacuum. For metal-carbon cluster systems (including M C + of Ti, Zr and V), their formation and the origin of delayed atomic ions were studied in a laser vaporization source coupled to a time-of-flight mass spectrometer. The mass spectrum of metal-carbon cluster ions (TiC2 and Zr C j+ cluster ions) obtained by using a titanium-zirconium (50 50) mixed alloy rod produced in a laser vaporization source (Nd YAG, X = 532 nm) and subsequently ionized by a XeCl excimer laser (308 nm) is shown in Figure 9.61. For cluster formation, methane ( 15% seeded in helium) is pulsed over the rod and the produced clusters are supersonically expanded in the vacuum. The mass spectrum shows the production of many zirconium-carbon clusters. Under these conditions only the titanium monomer, titanium dioxide and titanium dicarbide ions are formed. 

It had been expected, before the first macroscopic production and extraction of La Cs2 (Chai et al., 1991), that metallofullerenes based on the Cgo cage would be the most abundant metallofullerenes that were prepared in macroscopic amoimts, as was the case in empty fullerenes. This is simply because that Ceo is the most abundant fullerene which can be easily produced by either the arc-discharge or the laser furnace method (cf. Section 2.1). In fact, an earlier gas phase experiment on the production of carbon clusters containing La via the laser-vaporization cluster-beam technique (Heath et al., 1985) indicated that La Cgo is a prominent "magic number" species among various La C (44 < n < 80) clusters (Figure 1). 

Recent chemical experiments with transactinides have been carried out by application of refined methods. Fast transport is achieved by thermalizing the products of nuclear reactions recoiling out of the target in helium gas loaded with aerosol particles (e.g. KCl, M0O3, carbon clusters) of 10 to 200 nm on which the reaction products are adsorbed. Within about 2 to 5 s the aerosols are transported with the gas through capillary tubes over distances of several tens of metres with yields of about 50%. 

Fig. 2 Schematic diagram of the pulsed supersonic nozzle used to generate carbon cluster beams. The integrating cup can be removed at the indicated line. The vaporization laser beam (30-40 mJ at 532 nm in a 5-ns pulse) is focused through the nozzle, striking a graphite disk which is rotated slowly to produce a smooth vaporization surface. The pulsed nozzle passes high-density helium over this vaporization zone. This helium carrier gas provides the thermalizing collisions necessary to cool, react and cluster the species in the vaporized graphite plasma, and the wind necessary to carry the cluster products through the remainder of the nozzle. Free expansion of this cluster-laden gas at the end of the nozzle forms a supersonic beam which is probed 1.3 m downstream with a time-of-flight mass spectrometer.

One of the major applications of the CURES-EC procedure has been in calculating the electron affinities of the carbon clusters C [46]. Carbon clusters are common species in nature, since they are a product of combustion and contribute to environmental problems. They have been observed in the interstellar medium. The fuller-enes represent one of the most recent discoveries of a new form of carbon. Carbon clusters have many forms—linear chains, cyclic compounds and the fullerenes— because carbon can form covalent bonds. The NIST tables give 54 electron affinities values for C 14 for Si 17 for Ge 45 for Sn and 55 for Pb . Only the structures for the carbon compounds from n = 3 to 30 are identified [12]. 

The metal-carbon cluster systems we have considered so far in the present chapter, like the carboranes considered in the previous chapter, have contained one or more skeletal carbon atoms occupying vertex sites on the cluster deltahedron or deltahedral fragment. We now turn to some molecular cluster systems in which hypercoordinated carbon atoms occupy core sites in the middle of metal polyhedra. Most are metal carbonyl carbide clusters of typical formulae Mj (CO)yC. Their carbide carbon atoms are incorporated within polyhedra, which in turn are surrounded by y carbonyl ligands. Such compounds, for which few controlled syntheses are available, have been found primarily among the products of thermal decomposition of polynuclear metal carbonyls Mj (CO)j, their carbide carbon atoms result from disproportionation reactions of carbonyl ligands (2 CO CO2 + C). 

SCBD is a technique consisting in the production of a supersonic beam of inert gas seeded by carbon clusters (covalent aggregates with masses ranging from tens to thousands of atoms) by means of an appropriate... 

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). 

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