Carbon polyhedral

Tetracarbaboranes, which contain four carbon atoms in a single polyhedral skeleton, were rare until the discovery (129) of the metaHacarborane-mediated synthesis of (CH3)4C4BgHg [58815-26-2]. 

Fig. 9. A, Model for the apex of a carbon nanocone with a cone angle of 19.2 [94] B, polyhedral and spherical forms of a multiwall carbon particle formed from and...

Electron diffraction studies [3] have revealed that hexagons within the sheets are helically wrapped along the axis of the nanotubes. The interlayer spacing between sheets is 0.34 nm which is slightly larger than that of graphite (0.3354 nm). It was dso reported [2] that the helicity aspect may vary from one nanotube to another. Ijima et al. [2] also reported that in addition to nanotubes, polyhedral particles consisting of concentric carbon sheets were also observed. 

The carbon-arc plasma of extremely high temperatures and the presence of an electric field near the electrodes play important roles in the formation of nanotubes[ 1,2] and nanoparticles[3]. A nanoparticle is made up of concentric layers of closed graphitic sheets, leaving a nanoscale cavity in its center. Nanoparticles are also called nanopolyhedra because of their polyhedral shape, and are sometimes dubbed as nanoballs because of their hollow structure. 

Figure 2 illustrates a proposed growth process[3] of a polyhedral nanoparticle, along with a nanotube. First, carbon neutrals (C and C2) and ions (C )[16] deposit, and then coagulate with each other to form small clusters on the surface of the cathode. Through an accretion of carbon atoms and coalescence between clusters, clusters grow up to particles with the size fi-... 

For Sm, Eu, and Yb, on the other hand, nanocapsules containing carbides were not found in the cathode deposit by either TEM or XRD. To see where these elements went, the soot particles deposited on the walls of the reaction chamber was investigated for Sm. XRD of the soot produced from Sm203/C composite anodes showed the presence of oxide (Sm203) and a small amount of carbide (SmC2). TEM, on the other hand, revealed that Sm oxides were naked, while Sm carbides were embedded in flocks of amorphous carbon[12J. The size of these compound particles was in a range from 10 to 50 nm. However, no polyhedral nanocapsules encaging Sm carbides were found so far. 

Figure 6 Polyhedral view (to scale) of structure types ai(a), a2(b) and two orientations of 03(0 and d) for CtqMu. The muon is at the end of the dangling bond and in views (a) and (b) lies in the plane of the paper. For views (a), (b) and (c) four edge carbon atoms are also in the plane of the paper. The other visible atoms are above the paper. Each atom above the paper hides a corresponding atom below the paper except for type 03 where in the region of the muon the undistorted structure below the plane is shown with dashed lines. This is useful since it clearly shows the nature of the distortion. View (d) is an orientation of type 03 to illustrate that the distortion is similar to the other type a structures...

Apart from type 62, which is only slowly convergent to the optimised geometry, the other centres are well described by the ROHF method. Polyhedral views of the three type a structures are shown in Fig. 6. These all illustrate the change of hybridisation at the point of muonium attachment and at the adjacent carbon atom where the unpaired electron is effectively localised as expected from addition to an alkene. The bi and c defects (Fig. 7) are quite different. The expected hybridisation change to sp is clearly present for the atom bonded to muonium, but other significant distortions are not obvious. This is consistent with the prediction from resonance theory (Fig. 8) that the unpaired electron for these structures is delocalised over a large number of centres. 

Figure 1. Polyhedral structure of ECS-2 view along the [110] direction (left) dark gray small spheres are atoms of sodium. View of a six phenylene cage (right) with ethanol molecules inside the cage in the two statistical conformations light gray are oxygen, dark gray are carbon.

This chapter concerns carboranes (carbaboranes), which are boron clusters with at least one carbon atom as part of the polyhedral cage. Published studies on carboranes before 1981 were reviewed in GOMC (1982) and between 1982 and 1992 in COMC (1995). The present review covers the period of 1992 to early 2005. Unlike in previous chapters, boron hydrides with organic substituents attached to a boron atom, organopolyboron hydrides, are not discussed in this chapter. Borane clusters containing at least one non-carbon atom as part of the cage framework are reviewed in Chapters 3.03, 3.04 and 3.05 of this volume. 

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

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