Allotropes of

The first chemical preparation of cydo-heptasulfur S7 was reported by Schmidt et al. in 1968 who obtained solid S7 as intense yellow needle-like crystals [16]. 

The molecular structure of homocyclic S7 is of special interest insofar as it is not possible to construct a puckered ring in which the bond lengths, bond angles, and especially the dihedral angles typical for even membered rings such as S, Sg, and S12 are preserved [70]. A first X-ray structural analysis of solid 7 was attempted by Kawada and Hellner in 1970 who only derived a two-dimensional projection of the molecule. However, it was evident that the molecule must have a chair conformation and that the various dihedral angles of the molecule must differ significantly. 

Infrared and Raman spectroscopic studies have shown that S7 crystallizes in at least four different allotropic forms (a-, P-, y-, and 5-S7) [21]. Most likely all of the crystalline allotropes consist of the same type of heptamer. Detailed X-ray structural studies have proved this assumption for y-Sy and (5-S7 [20, 71]. In solution S7 undergoes rapid pseudorotation, i.e., the ring atoms become equivalent on average (observed by Raman spectroscopy and by NMR spectroscopy in the case of the related molecule 1,2-86285) 

7-87 crystallizes similar to a-Sy, though under special conditions y-87 can be obtained as single crystals in the monochnic space group Pli/c-Cyh [20]. 

S-Sy forms block-shaped, tetragonal-bipyramidal and sarcophagus-hke single crystals of the monoclinic space group P2iln-C2] [20, 21,71]. 

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EttI R, Chao I, Diederich F and Whetten R L 1991 Isolation of C-g, a chiral (Dg) allotrope of carbon Nature 353... 

The eommonest erystalline forms of earbon, cubie diamond and hexagonal graphite, are elassical examples of allotropy that are found in every chemistry textbook. Both diamond and graphite also exist in two minor crystallographie forms hexagonal diamond and rhombohedral graphite. To these must be added earbynes and Fullerenes, both of which are crystalline earbon forms. Fullerenes are sometimes referred to as the third allotrope of carbon. However, sinee Fullerenes were diseovered more recently than earbynes, they are... 

Chapter 1 contains a review of carbon materials, and emphasizes the stmeture and chemical bonding in the various forms of carbon, including the foui" allotropes diamond, graphite, carbynes, and the fullerenes. In addition, amorphous carbon and diamond fihns, carbon nanoparticles, and engineered carbons are discussed. The most recently discovered allotrope of carbon, i.e., the fullerenes, along with carbon nanotubes, are more fully discussed in Chapter 2, where their structure-property relations are reviewed in the context of advanced technologies for carbon based materials. The synthesis, structure, and properties of the fullerenes and... 

Over 100 years ago it had been demonstrated that ozone (Oj). the unstable triatomic allotrope of oxygen, could destroy molds and bacteria and by 1892 several experimental ozone plants were in operation in Europe. In the 1920s, however, as a result of wartime research, during World War I, chlorine became readily... 

Carbon nanotubes (CNTs) as well as fullerenes are splendid gift brought to the Earth from the red giant carbon stars in the long-distant universe through the spectroscopy. Moreover, those belong to new carbon allotropes of the mesoscopic scale with well-defined structures. In particular, CNTs are considered to be the materials appropriate to realise intriguing characteristics related to the mesoscopic system based on their size and physicochemical properties. 

Boron is unique among the elements in the structural complexity of its allotropic modifications this reflects the variety of ways in which boron seeks to solve the problem of having fewer electrons than atomic orbitals available for bonding. Elements in this situation usually adopt metallic bonding, but the small size and high ionization energies of B (p. 222) result in covalent rather than metallic bonding. The structural unit which dominates the various allotropes of B is the B 2 icosahedron (Fig. 6.1), and this also occurs in several metal boride structures and in certain boron hydride derivatives. Because of the fivefold rotation symmetry at the individual B atoms, the B)2 icosahedra pack rather inefficiently and there... 

Quite apart from the fullerene cluster molecules, numerous other molecular allotropes of carbon, C , have been discovered in the gases formed by the laser vaporization/supersonic expansion of graphite. The products are detected by mass... 

Ozone, O3, is the triatomic allotrope of oxygen. It is an unstable, blue diamagnetic gas with a characteristic pungent odour indeed, it was first detected by means of its smell, as reflected by its name (Greek o eiv, ozein, to smell) coined by C. F. Schonbein in 1840. Ozone can be detected by its smell in concentrations as low as 0.01 ppm the maximum permissible concentration for continuous exposure is 0.1 ppm but levels as high as 1 ppm are considered non-toxic if breathed for less than 10 min. 

Figure 16.1 Structures of various allotropes of selenium and the structure of crystalline tellurium (a) the Seg unit in a- fi- and y-red selenium (b) the helical Se chain along the c-axis in hexagonal grey selenium (c) the similar helical chain in crystalline tellurium shown in perspective and (d) projection of the tellurium structure on a plane perpendicular to the c-axis.

In the solid state all three elements have typically metallic structures. Technetium and Re are isostructural with hep lattices, but there are 4 allotropes of Mn of which the o-fomi is the one stable at room temperature. This has a bcc structure in which, for reasons which are not clear, there are 4 distinct types of Mn atom. It is hard and brittle, and noticeably less refractory than its predecessors in the first transition series. 

Much current research is centering on polyynes—linear carbon chains of sp-hybridized carbon atoms. Polyynes with up to eight triple bonds have been detected in interstellar space, and evidence has been presented for the existence of carbyne, an allotrope of carbon consisting of repeating triple bonds in long chains of indefinite length. 

Allotropy is a very common phenomenon shown by metals, metalloids, and nonmetals. In the gaseous or liquid state, allotropes most often differ from one another in molecular formula. Consider, for example, the two allotropes of gaseous oxygen, 02, and ozone, 03. 

The two most familiar allotropes of sulfur, rhombic and monoclinic, have the same molecular formula, S8. However, they differ in crystal structure. Using the phase diagram shown in Figure C. you can deduce how to convert either of these allotropes to the other. Notice that rhombic sulfur is the stable allotrope at temperatures below about 95°C. If it is heated to that temperature at... 

Pencil lead" is almost pure graphite. Graphite is the stable elemental form of carbon at 25°C and 1 atm. Diamond is an allotrope of graphite. Given... 

Allotrope One of two or more forms of an element in the same physical state. Graphite and diamond are allotropes of carbon 02 and 03 are allotropes of oxygen, 250... 

What I hope to have added to the discussion has been a philosophical reflection on the nature of the concept of element and in particular an emphasis on elements in the sense of basic substances rather than just simple substances. The view of elements as basic substances, is one with a long history. The term is due to Fritz Paneth, the prominent twentieth century radio-chemist. This sense of the term element refers to the underlying reality that supports element-hood or is prior to the more familiar sense of an element as a simple substance. Elements as basic substances are said to have no properties as such although they act as the bearers of properties. I suppose one can think of it as a substratum for the elements. Moreover, as Paneth and before him Mendeleev among others stressed, it is elements as basic substances rather than as simple substances that are summarized by the periodic table of the elements. This notion can easily be appreciated when it is realized that carbon, for example, occurs in three main allotropes of diamond, graphite and buckminsterfullenes. But the element carbon, which takes its place in the periodic system, is none of these three simple substances but the more abstract concept of carbon as a basic substance. 

Buckminsterfullerene is an allotrope of carbon in which the carbon atoms form spheres of 60 atoms each (see Section 14.16). In the pure compound the spheres pack in a cubic close-packed array, (a) The length of a side of the face-centered cubic cell formed by buckminsterfullerene is 142 pm. Use this information to calculate the radius of the buckminsterfullerene molecule treated as a hard sphere, (b) The compound K3C60 is a superconductor at low temperatures. In this compound the K+ ions lie in holes in the C60 face-centered cubic lattice. Considering the radius of the K+ ion and assuming that the radius of Q,0 is the same as for the Cft0 molecule, predict in what type of holes the K ions lie (tetrahedral, octahedral, or both) and indicate what percentage of those holes are filled. 

Robert Curl, Richard Smalley, and Harold Kroto were awarded the Nobel prize in chemistry in 1996 for the discovery of the soccer-ball-shaped molecule C60. This fundamental molecule was the first of a new series of molecular allotropes of carbon. The enthalpy of combustion of C60 is —25 937 kj-mol, and its enthalpy of sublimation is +233 kj-mol There are 90 bonds in C60, of which 60 are single bonds and 30 are double bonds. Like benzene, C60 has... 

Solid carbon exists as graphite, diamond, and other phases such as the fullerenes, which have structures related to that of graphite. Graphite is the thermodynamically most stable of these allotropes under ordinary conditions. In this section, we see how the properties of the different allotropes of carbon are related to differences in bonding. 

Distinguish the allotropes of carbon by their structures and show how their structures affect their properties. 

An allotrope of oxygen, ozone, 03 (8), is formed in the stratosphere by the effect of solar radiation on 02 molecules. Its total abundance in the atmosphere is equivalent to a layer that, at the ordinary conditions of 25°C and 1 bar, would cover the Earth to a thickness of only 3 mm, yet its presence in the stratosphere is vital to the maintenance of life on Earth (see Box 13.3). Ozone can be made in the laboratory by passing an electric discharge through oxygen. It is a blue gas that... 

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