Isomeric carbon clusters

This review will restrict itself to boron-carbon multiple bonding in carbon-rich systems, as encountered in organic chemistry, and leave the clusters of carboranes rich in boron to the proper purview of the inorganic chemist. Insofar as such three-dimensional clusters are considered at all in these review, interest will focus on the carbon-rich carboranes and the effect of ring size and substituents, both on boron and carbon, in determining the point of equilibrium between the cyclic organoborane and the isomeric carborane cluster. A typical significant example would be the potential interconversion of the l,4-dibora-2,5-cyclohexadiene system (7) and the 2,3,4,5-tetracarbahexaborane(6) system (8) as a function of substituents R (Eq. 2). 

Among the many carbon clusters identified spectroscopically (from C40 to C2oo)> a few have been structurally characterized. For instance, Figure 12 illustrates the molecular structures of C60, C70, C76, C78, C80 and C82 (in their most stable isomeric forms).9... 

Yi, J-Y. Bemholc, J. 1992 Isomerization of C6 fullereness. J. chem. Phys. 96, 8634-8636. Zhang, Q-L. et at. 1986 Reactivity of large carbon clusters spheroidal carbon shells and their possible relevance to the formation and morphology of soot. J. phys. Chem. 90, 525-528. 

We have recently developed a gas-phase ion chromatography technique and applied it to carbon cluster cations " " and anions""". A pulse of mass-selected cluster ions is injected into a high-pressure drift cell filled with 2-5 torr of helium. The ionic mobilities of different isomeric structures depend on their different collision cross-sections with He, and the isomers are therefore separated while drifting through the cell, under the influence of a weak electric field. The absolute value of the ionic mobility for a given cluster together with computer simulations often allows unambiguous determination of the cluster... 

We found that major products of Cgo suspended in hexane and methanol under laser ablation are graphite-like carbon. It probably arises from photodecomposition of fullerene network and photochemical and/or thermal isomerization from fullerene structure to graphite-like one. Another formation route of graphite-like carbon is clustering of small carbon clusters. 

Much work was, and still is, being done on the properties of this structure, on its possible isomerism, and on the properties of many other similar structures of varying size. In parallel, there arose the idea—it is difficult to know exactly when—that a sphere might not be the only hollow topological shape that a carbon cluster might assume, and interest in such possibilities has grown, steadily rather than explosively. ... 

The various modes of bonding that have been observed for alkenes to the trinuclear osmium clusters are shown in Fig. 7 [see (88)]. The simple 77-bonded structure (a) is relatively unstable and readily converts to (c) the vinyl intermediate (b) is obtained by interaction of alkene with H2Os3(CO)10 and also readily converts to (c) on warming. Direct reaction of ethylene with Os3(CO)12 produces (c), which is considered to be formed via the sequence (a) — (b) — (c) and (d). Both isomers (c) and (d) are observed and involve metal-hydrogen and metal-carbon bond formation at the expense of carbon-hydrogen bonds. In the reaction of Os3(CO)12 with C2H4, the complex 112088(00)902112, (c), is formed in preference to (d). Acyclic internal olefins also react with the carbonyl, with isomerization, to yield a structure related to (c). Structure (c) is... 

Similar reactions applied to transition metal-acetylene complexes appear capable of separating the 2 carbon atoms originally linked by the acetylenic triple bond 18). Thermal isomerization of metal-acetylene complexes may achieve the same result, showing how metal clusters can catalyze scrambling reactions of acetylenes, e.g.. 

The product of acetyl-CoA carboxylase reaction, malonyl-CoA, is reduced via malonate semialdehyde to 3-hydroxypropionate, which is further reductively converted to propionyl-CoA. Propionyl-CoA is carboxylated to (S)-methylmalonyl-CoA by the same carboxylase. (S)-Methylmalonyl-CoA is isomerized to (R)-methylmal-onyl-CoA, followed by carbon rearrangement to succinyl-CoA by coenzyme B 12-dependent methylmalonyl-CoA mutase. Succinyl-CoA is further reduced to succinate semialdehyde and then to 4-hydroxybutyrate. The latter compound is converted into two acetyl-CoA molecules via 4-hydroxybutyryl-CoA dehydratase, a key enzyme of the pathway. 4-Hydroxybutyryl-CoA dehydratase is a [4Fe-4S] cluster and FAD-containing enzyme that catalyzes the elimination of water from 4-hydroxybutyryl-CoA by a ketyl radical mechanism to yield crotonyl-CoA [34]. Conversion of the latter into two molecules of acetyl-CoA proceeds via normal P-oxidation steps. Hence, the 3-hydroxypropionate/4-hydroxybutyrate cycle (as illustrated in Figure 3.5) can be divided into two parts. In the first part, acetyl-CoA and two bicarbonate molecules are transformed to succinyl-CoA, while in the second part succinyl-CoA is converted to two acetyl-CoA molecules. 

Figure 2. GC-FID chromatograms for the sulfide fractions from different Alberta petroleums. The peaks labeled B13 and B20 correspond to the bicyclic terpenoid sulfides with 13 and 20 carbons, respectively. The peak labeled T23 corresponds to the tetracyclic terpenoid sulfide with 23 carbons and peaks due to the hopane sulfides are indicated at the end of the chromatograms. The clusters of peaks spaced one carbon apart on the Bellshill Lake trace correspond mainly to complex mixtures of isomeric monocyclic sulfides possessing a linear carbon framework. These sulfides have been removed by biodegradation from the upper two samples. For more complete peak identification see References 9, 10 and 35. (Reproduced from Reference 34. Copyright 1989, American Chemical Society.)...

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