Tetrahedral Atoms

Freyhardt C C, Tsapatsis M, Lobe R F, Balkus Jr K J and Davis M E 1996 A high-silica zeolite with a 14-tetrahedral-atom pore opening Nature 381 295-8... 

Fig. 9.76 The stereochemistry about tetrahedral atoms can be maintained with an appnrpriate chiral constraint.

The most common cause of chirality is the presence of four different substituents bonded to a tetrahedral atom, but that atom doesn t necessarily have to be carbon. Nitrogen, phosphorus, and sulfur are all commonly encountered in organic molecules, and all can be chirality centers. We know, for instance, that trivalent nitrogen is tetrahedral, with its lone pair of electrons acting as the fourth "substituent" (Section 1.10). Is trivalent nitrogen chiral Does a compound such as ethylmethylamine exist as a pair of enantiomers ... 

Thus we have shown that when s and p orbitals are available and s—p quantization is broken an atom can form four (or fewer) equivalent bonds which are directed towards tetrahedron corners. To the approximation involved in these calculations the strength of a bond is independent of the nature of other bonds. This result gives us at once the justification for the tetrahedral carbon atom and other tetrahedral atoms, such as silicon, germanium, and tin in the diamond-type crystals of the elements and, in general, all atoms in tetrahedral structures. 

Observed distances between tetrahedral atoms in other crystals, given in Table V, are also in satisfactory agreement with the radius sums. One interesting crystal is Various investigators have suggested different... 

In other crystals an octahedral metal atom is attached to six non-metal atoms, each of which forms one, two, or three, rather than four, bonds with other atoms. The interatomic distance in such a crystal should be equal to the sum of the octahedral radius of the metal atom and the normal-valence radius (Table VI) of the non-metal atom. This is found to be true for many crystals with the potassium chlorostannate (H 61) and cadmium iodide (C 6) structures (Table XIB). Data are included in Table XIC for crystals in which a tetrahedral atom is bonded to a non-metal atom with two or three covalent bonds. The values of dcalc are obtained by adding the tetrahedral radius for the former to the normal-valence radius for the latter atom. 

The stereochemistry at the migration origin A is less often involved, since in most cases it does not end up as a tetrahedral atom but when there is inversion here, there is an Sn2 type process at the beginning of the migration. This may or may not be accompanied by an Sn2 process at the migration terminus B ... 

Fig. 7.19 (a) Ambiguous 2D drawings of stereo centers, (b) Recommended encoding of tetrahedral atoms and double bonds. 

Molecules with a planar coordination figure do not contain planar atoms . Further, tetrahedral atoms , chiral atoms etc. are nonsense. A minimum of four atoms is required for a chiral structure. 

A tetrahedral atom with four different groups attached to it is a stereocenter (chiral center, stereogenic center)... 

Stereocenter any tetrahedral atom with four different groups attached to it. 

Figure 5.22 Enantiomeric forms of 1,3-dichloroallene. These two molecules are nonsuperposable mirror images of each other and are therefore chiral. They do not possess a tetrahedral atom with four different groups, however.

Figure 11. The truncated octahedron building block (also termed sodalite cage,f or p-cage ) (a) tetrahedral atoms (usually Si or Al) are located at the corners of the polygons with oxygen atoms halfway between them. Illustration of the linkage, through double four-membered rings, of two truncated octahedra (b) and the structure of zeolite-A (c).

Fig. 8. Polyhedral representation of the Keggin structure (central tetrahedral atom not shown). The lines seen in the center are the edges of the V30i3 subunit at the back (cf. also Fig. 21). Black dots indicate where one of the two five-coordinate capping units are attached in [Vi5042]9 .

Hanson, K. R. Applications of the sequence rule. I. Naming the paired ligands g,g at a tetrahedral atom Xggij. II. Naming the two faces of a trigonal atom Yghi. J. Am. Chem. Soc. 88, 2731-2742 (1966). 

Estermann, M., McCusker, L.B., Baerlocher, C., Merrouche, A., and Kessler, H. (1991) A synthetic gallophos-phate molecular sieve with a 20-tetrahedral-atom pore opening. Nature, 352, 320-323. 

Some vibrational modes of zeolites are sensitive to the amount of aluminum in the framework [93]. The substitution of aluminum for silicon atoms in the zeolite framework changes the T-O-T bond angles (where T is a tetrahedral atom that can be either Si or Al). This is primarily due to the smaller size and different charge density of the aluminum atoms compared to silicon. This results in a shift in frequency for vibrational modes in the zeolite due to external linkages. The T-O-T asymmetric (1100-980 cm ) and symmetric (800-600 cm ) stretching modes as well as structural unit vibrations Mke double four- and double six-rings exhibit a shift to lower frequencies as the aluminum content of the framework is increased. Figure 4.19 shows this relationship for a faujasite-type framework. 

A stereochemical condition that describes a stereoheterotopic group C in the compound XABC2 containing a prochiral tetrahedral atom X having substituents A, B, C, and C. 

Virginia Polytechnic Institute number 5 (VPI-5) is a family of aluminophosphate based molecular sieves (refs. 1-5) which share a common three-dimensional topology and contain 18-membered rings (refs. 1-8). The extra-large pores of the VPI-5 materials are unidimensional channels circumscribed by rings containing 18 tetrahedral atoms and possess free diameters of approximately 12-13 A. 

Interconversions of acychc carbon-centered radicals between n and a types are low-energy processes. The methyl radical is planar, but increasing alkyl substitution at the radical center results in an increasing preference for pyramidalization. The ferf-butyl radical is pyramidalized with the methyl groups 10° from planarity (the deviation from planarity for a tetrahedral atom is 19°) and a barrier to inversion of 0.5 kcal/mol. When a radical center is in a carbocycle, a planar radical is favored for all cases except the cyclopropyl radical, and the barrier for inversion in cyclopropyl is only 3 kcal/mol. ... 

A molecule is chiral if it is not superimposable on its mirror image. A tetrahedral atom with four different groups is just one of the factors that confer chirality on a molecule. There are a number of molecules where a tetrahedral atom with four different groups is not present, yet they are not superimposable, i.e. chiral. For example, 1,3-dichloroallene is a chiral molecule, hut it does not have a tetrahedral atom with four different groups. 

A decision cannot be made between structures A, B, and C, which are, indeed, so closely similar in nature that there can be no large energy difference between them. Moreover, they satisfy the other conditions for resonance they involve the same number of unpaired electrons (zero), and they correspond to about the same equilibrium configuration of the nuclei (linear, for a central tetrahedral atom forming either two double bonds or a single bond and a triple bond). We accordingly expect the normal state of the molecule to correspond to resonance among structures A, B, and C, with small contributions by the other leas... 

---