Tetrahedrally coordinated carbon atoms

Protonation of 4b leads to the symmetrically substituted 3b (Scheme 3.2-3) and methylation of 4b at temperatures higher than —60 °C gives 3c (Scheme 3.2-5) [19]. In the latter reaction, 6a can be identified as an intermediate at —80 °C by 13C NMR spectroscopy [19]. Its planar-tetracoordinate carbon atom is strongly de-shielded 3 13 C = 144 ppm) as compared with tetrahedrally-coordinated carbon atoms connected to three boron and one silicon center (d 13C = 70-100 ppm). Computations for the model compounds 6A and 6B give 144 and 104 ppm, re-... 

The diamond structure is isotropic (the same in all principal directions), with tetrahedrally coordinated carbon atoms. The carbon-carbon distance is what we would expect for single bonds (154 pm). Diamond is transparent, an electrical insulator, refractory, the hardest substance known, and is used in cutting tools. Diamond s isotropic three-dimensional structure explains its hardness it is an electrical insulator, but interestingly it has the highest thermal conductivity of any known substance (approximately five times that of copper), which is why diamond... 

Diamondoids, when in the solid state, melt at much higher temperatures than other hydrocarbon molecules with the same number of carbon atoms in their structures. Since they also possess low strain energy, they are more stable and stiff, resembling diamond in a broad sense. They contain dense, three-dimensional networks of covalent bonds, formed chiefly from first and second row atoms with a valence of three or more. Many of the diamondoids possess structures rich in tetrahedrally coordinated carbon. They are materials with superior strength-to-weight ratio. 

Like many other chemical concepts the concept of strain is only semi-quantitative and lacks precise definition. Molecules are considered strained if they contain internal coordinates (interatomic distances (bond lengths, distances between non-bonded atoms), bond angles, torsion angles) which deviate from values regarded as normal and strain-free . For instance, the normal bond angle at the tetra-coordinated carbon atom is close to the tetrahedral value of 109.47°. In the course of force field calculations these normal values are defined more satisfactorily, though in a somewhat different way, as force field parameters. 

The X-ray crystal structure of Pb2(o-tolyl)6 shows one centrosymmetric molecule per unit cell. Figure 53 clearly shows that there is no expansion of coordination of the lead atom in Pb2(o-tolyl)6- The bond distances of the tetrahedral coordinated lead atom to the carbon atoms are in the range of 2.242-2.249 A the Pb—Pb distance was found to be... 

Like a single carbon atom capped with tetrahedrically coordinated hydrogen atoms, Fig. 4.4, a cluster of sp -bonded carbon atoms can also be capped with hydrogen to form hydrogenated fragments of a diamond structure diamondoids. 

The intrinsic stability of the aromatic n system has two major consequences for the course of reactions involving it directly. First, the aromatic ring is less susceptible to electrophilic, nucleophilic, and free-radical attack compared to molecules containing acyclic conjugated n systems. Thus, reaction conditions are usually more severe than would normally be required for parallel reactions of simple olefins. Second, there is a propensity to eject a substituent from the tetrahedral center of the intermediate in such a way as to reestablish the neutral (An + 2)-electron system. Thus, the reaction is two step, an endothermic first step resulting in a four-coordinate carbon atom and an exothermic second step, mechanistically the reverse of the first, in which a group is ejected. The dominant course is therefore a substitution reaction rather than an addition. 

Figure 1. Top row staggered conformation of ethane row B the three conformers of -butane (only carbon atoms are shown in this and subsequent drawings) row C the four orientations of vectors (grid coordinates a-d) around the tetrahedral central carbon atom of neopentane row D one of the chiral gauche rotamers, and the achiral anti rotamer of -butane row E chair-cyclohexane, and its Cartesian coordinates X, y, z.

Several models are presently under discussion to explain the formation of highly tetrahedrally coordinated carbon ta-C. According to one of these theories the ta-C deposition can be described as a subplantation process [19, 29]. The principle of this process is that carbon ions penetrate the first atomic layers and enter interstitial positions, thereby increasing the local density and inducing local compressive... 

Figure 29.6 The structure of [Hg(>j -QH4PPh3)l2]2 showing the essentially tetrahedral coordination of the mercury atoms and of the carbon atoms attached to them.

One important difference between the present and the previous case should be noted. For the hydroquinone clathrates, where the wall of a cavity consists of 12 OH groups, 6 adjacent carbon atoms, and 6 CH groups in ortho position to the OH groups, it seemed best to consider the product z qjk) as one unknown. For hydrates one may not do this the walls of both types of cavities consist exclusively of tetrahedrally-coordinated water molecules. Hence, one should use the same value of (,eg/k) —characteristic for a water molecule in a hydrate lattice—for both types of cavities and multi-... 

The Tetrahedral Carbon Atom.—We have thus derived the result that an atom in which only s and p eigenfunctions contribute to bond formation and in which the quantization in polar coordinates is broken can form one, two, three, or four equivalent bonds, which are directed toward the corners of a regular tetrahedron (Fig. 4). This calculation provides the quantum mechanical justification of the chemist s tetrahedral carbon atom, present in diamond and all aliphatic carbon compounds, and for the tetrahedral quadrivalent nitrogen atom, the tetrahedral phosphorus atom, as in phosphonium compounds, the tetrahedral boron atom in B2H6 (involving single-electron bonds), and many other such atoms. 

As in organic chemistry, there are several sources of chirality at a metal center. As for an asymmetric carbon atom in an organic molecule, the coordination of the metal ion by four different monodentate hgands in a tetrahedral con-... 

The great majority of known chiral compounds are naturally occurring organic substances, their molecules having one or more asymmetrically substituted carbon atoms (stereogenic atoms). Chirality is present when a tetrahedrally coordinated atom has... 

The unit cell of cubic diamond corresponds to a face-centered packing of carbon atoms. Aside from the four C atoms in the vertices and face centers, four more atoms are present in the centers of four of the eight octants of the unit cell. Since every octant is a cube having four of its eight vertices occupied by C atoms, an exact tetrahedral coordination results for the atom in the center of the octant. The same also applies to all other atoms — they are all symmetry-equivalent. In the center of every C-C bond there is an inversion center. As in alkanes the C-C bonds have a length of 154 pm and the bond angles are 109.47°. 

The zinc atom has almost ideal tetrahedral coordination geometry, with bond angles ranging from 115.37(10)° to 123.60(9)°. The zinc-carbon bond (1.964(3) A) has a similar length as in the neutral [MeZn(OBut)]4 tetramer,106 in which zinc is also tetrahedrally surrounded by one alkyl group and three oxygen atoms. 

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