Platonic tetrahedron

It often is noted that van t Hoff made the apparently "positivist" statement that "the representations themselves, atom, molecule, their dimensions, and perhaps their shapes, are after all something doubtful, as is the tetrahedron itself. "103 However, as the theoretical chemist Roald Hoffmann has aptly suggested, this does not necessarily mean that van t Hoff doubted "chemistry in space" but perhaps that he did not feel committed to the exact geometrical form that is the Platonic tetrahedron, a form that is not only idealized but static. 104... 

D2h isomer of Kuratowski s cyclophane (116) [153, 154], where branching is introduced within the bridges, shows a cylindrical graph with an internal cross diameter connection. Okazaki et al. s lantern-shaped molecule (117) [155, 156] approximates a pyramidal shape, a high symmetric structure related to the Platonic tetrahedron. Platonic and other highly symmetrical graphs are compiled in the next section. 

You say that your nonlinear molecule has the high symmetiy of a regular polyhedron, such as a tetrahedron, cube, octahedron, dodecahedron, icosahedron,... sphere. If it is a sphere, it is monatomic. On the other hand, if it is not monatomic, it has the symmetry of one of the Platonic solids (see the introduction to Chapter 8). 

Figure 2.13. The dodecahedron and the icosahedron are two of the five Platonic solids (regular polyhedra), the others being the tetrahedron, the cube, and the octahedron, (a) The dodecahedron has twelve regular pentagonal faces with three pentagonal faces meeting at a point, (b) The icosahedron has twenty equilateral triangular faces, with five of these meeting at a point.

Figure B.2 shows polyhedra commonly encountered. The five Platonic (or regular) solids are shown at the top. Beside the octahedron and cube, the octahedron is shown inside a cube, oriented so the symmetry elements in common coincide. These solids are conjugates one formed by connecting the face centers of the other. The tetrahedron is its own conjugate, because connecting the face centers gives another tetrahedron. The icosahedron and pentagonal dodecahedron are conjugates. The square antiprism and trigonal...

Of course an icosahedron is not the only three dimensional design that can form a capsule. MacGillivray and Atwood proposed a structural classification for supramolecular assemblies based on the five Platonic and 13 Archimedean solids [21], The Platonic solids, illustrated in Fig. 3.10, are the tetrahedron, cube, octahedron, dodecahedron and icosahedron. 

Fig. 3.10 The Platonic solids (left to right) tetrahedron, cube, octahedron, dodecahedron and icosahedron...

The belief that molecular-level prototypes can be constructed to imitate familiar macroscopic objects has been the inspiration for many intensive and successful synthetic efforts [51-57], Molecular-level construction of Platonic solids such as the tetrahedron [54], the cube [55], and the dodecahedron [56], as well as non-Euclidian objects such as molecular knots [57a], Mobius strips [57b], and even molecular machinery [57d] exemplifies such efforts (cf. Chapter 5 of this book). Synthetic organic chemists have developed a rich legacy in this area as described in more detail by Nickon and Silversmith [57c]. 

There are only five regular convex polyhedra, a very small number indeed. The regular convex polyhedra are called Platonic solids because they constituted an important part of Plato s natural philosophy. They are the tetrahedron, cube (hexahedron), octahedron,... 

The roots of molecular beauty can be traced back to the Platonic tradition. To Plato, the most beautiful bodies in the whole realm of bodies were the tiny polyhedra, now deemed the Platonic solids, which he proposed comprise the universe the four elements - earth (cube), fire (tetrahedron), air (octahedron), water (icosahedron) - and the ether (dodecahedron) (Fig. 1). Joachim Schummer, who has written [9] extensively on chemical aesthetics, writes ... 

The five fundamental solids, the tetrahedron, the octahedron, the icosahedron and the dodecahedron were known to the Ancient Greeks. Constructions based on isosceles triangles are described for the first four by Plato in his Dialogue Timaeus, where he associated them with fire, earth, air, water and noted the existence of the fifth, the dodecahedron, standing for the Universe as a whole. These five objects are now known as the Platonic solids — defined as the convex polyhedra because they exhibit equivalent convex regular polygonal faces. 

Molecular frameworks with the shapes of each Platonic solid are known. In symmetry terms, the Platonic solids split into two families the tetrahedron, the cube and the octahedron, which have cubic symmetry, and the icosahedron and the dodecahedron, which have icosahedral symmetry. 

These are groups which contain more than one threefold or higher axis. We will limit our consideration to the symmetry groups which describe the Platonic solids Td for the regular tetrahedron, Oh for the cube and regular octahedron, I/, for the regular dodecahedron and icosahedron, and JCh for the sphere. Some molecules in the cubic groups are shown below ... 

Viral capsid structures are built with high efficiency and economy from a single fundamental unit and as such are analogous to geodesic domes. There are five regular polyhedra, a tetrahedron, a cube, an octahedron, a dodecahedron and an icosaliedron. These polyhedra are often referred to as Platonic solids because of their significance in Plato s natural... 

FIGURE 15.23. Platonic figures found in virus structures, and their symmetries (see Ref. 191). (a) Tetrahedron, (b) octahedron, and (c) icosahedron. Rotation axes are indicated. 

Platonic solid Any one of five regular three-dimensional solids - the tetrahedron, the cube, the octahedron, the dodecahedron, and the icosahedron. 

For nearly 30 years, the field of metal clusters has provided chemists a vast arena in which to work. Early results were often surprising. Crystallographic analysis revealed that compounds, such as Os3(CO)j2 and Rh6(CO),g, were often incorrectly formulated by traditional techniques. " It was quickly evident that a large number of ligand-stabilized metal clusters could be synthesized, and extensive exploratory research opened a rich, interesting field. Discovery of unprecedented structural features was the norm and remains common. The fact that each platonic solid (namely, the tetrahedron, octahedron, cube, icosahedron, and pentagonal dodecahedron) is now represented in transition metal cluster chemistry illustrates the structural variety present in this class of compounds. A short preview of particular metal clusters whose geometries approximately conform to these Platonic solids provides an introduction to some of the structural phenomena considered elsewhere in this chapter. 

Three or more molecules may assemble in the solid state to form a finite assembly with connecting forces propagated in 3D. The components of such an assembly will typically form a polyhedral shell. The shell may accommodate chemical species as guests. The polyhedron may be based on a prism or antiprism, as well as one of the five Platonic (e.g. cube, tetrahedron) or 13 Archimedean (e.g. truncated tetrahedron) solids.4... 

The Greek philosopher Pythagoras (circa 582-500 b.c.) and his students studied the regular polyhedra and introduced them into the Pythagorean cosmology as the symbols of the five elements the tetrahedron for fire, the cube for earth, the octahedron for air, the icosahedron for water, and the dodecahedron for ether (plate 7). Plato (427-347 B.c.) and the members of his school discussed the regular polyhedra with such vigor as to have caused them to be called the Platonic solids for over 2,300 years. 

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