Tetrahedrane molecule

Fig, 2.4. Relief map of ihe electronic charge density in the tetrahedrane molecule, C4H4. The plane shown is a 0-4 symmetry plane and contains two carbon nuclei and their associated protons. The charge density at the central critical point is a local minimum with a value of 0.165 au. The two-dimensional maximum in the foreground is the (2, — 2) maximuln in p in the interatomic surface of the out-of-plane carbon nuclei. The value of p at this point is 0.246 au. A contour map of the charge density in the same plane is shown in Fig. 2.10. 

Molecules such as P4 and the polyanionic clusters such as Si4- or As2- that are discussed in Section 13.2 are representatives of electron precise closo clusters. Organic cage molecules like tetrahedrane (C4R4), prismane (C6H6), cubane (C8H8), and dodecahedrane (C20H20) also belong to this kind of cluster. 

For the practical design of hypersurfaces, i.e. cuts through the (3n-6)dimensional hyperspace, some hints are outlined. The main purpose, however, is to illustrate the usefulness of hypersurface calculations especially for the detection, identification and characterization of unstable molecules. Examples chosen comprise the structure of RS-C=C-SR, the relative stability of thioacroleine isomers C,H S, the structural changes accompanying the oxidation of hydrazine and some of its derivatives, the isomerization of tetrahedrane to cyclobutadiene both thermally as well as on oxidation, the predicted existence of F SS and nonexistence of CI2SS or H2SS, and, finally, some aspects of the thermal decomposition of methyl and vinyl azides. 

Also, there is an esthetic element mixed into this motivation, which probably is derived from the tradition of stripped-down test molecules in physical organic chemistry, in which studies of the parent member of a series tend to be valued more highly than those of its derivatives. One thinks, for example, of the emphasis placed on studies of the unsubstimted molecules methylene, norbomyl cation, cyclobutadiene, tetrahedrane, benzyne, and so on. The higher valuation also is associated in some cases with the formidable difficulties experienced by experimentalists in the synthesis and observation of these species. 

In some cases, steric interactions can prevent unimolecular reactions. Tetrahe-drane (18) has been the subject of a number of studies, and the conclusion is that, if formed, it would rapidly decompose to form two molecules of acetylene. However, tetra-tert-butyltetrahedrane (19) is a quite stable substance, and on heating rearranges to tetra-tert-butylcyclobutadiene. An orbital symmetry " analysis of the cleavage of tetrahedrane to acetylene indicates that it involves a torsional motion that in the case of the tert-butyl substituted derivative would bring the tert-butyl groups very close to each other. As a result, this mode of reaction is not possible, and the compound is relatively stable. 

From time to time throughout the book we have spread before your eyes some wonderful structures. Some have been very large and complicated (such as palytoxin, p. 19) and some small but difficult to believe (such as tetra-f-butyl tetrahedrane, p. 373). They all have one thing in common. Their structures were determined by spectroscopic methods and everyone believes them to be true. Among the most important organic molecules today is Taxol, an anticancer compound from yew trees. Though it is a modern compound, in that chemists became interested in it only in the 1990s, its structure was actually determined in 1971. 

This molecule now has three-, four-, and five-membered rings fused together in a tricyclic cage structure. This is nowhere near the limit for cage molecules. You saw tetra-t-butyl tetrahedrane in Chapter 15, and you will see in Chapter 37 how even molecules such as cubane can be made. 

Tetrahedrane, (CH)4, would be the simplest regular polyhedral polycyclic hydrocarbon (Figure 3-21a). However, since it has such a high strain energy, it has not (yet ) been prepared in spite of considerable efforts [46], By now, over 10 different derivatives of tetrahedrane have been prepared, for example, tetra-/c/7-butyltetrahedrane (Figure 3-21b) [47], It is amazingly stable, perhaps because the substituents help clasp the molecule together. 

It reduces to ct + e. Again, the ordering is different, but the similarity between their electronic structure is obvious. A series of molecules and their similarities are illustrated in Figure 7-30. The first molecule is tetrahedrane and the last one is a cluster with metal-metal bonds which can be considered as being the inorganic analog of tetrahedrane. 

Tetrahedranes belong to the most strained molecules. The molecular structure of tetra- r butyltetrahedrane (209) has been determined in its low-temperature modification at 213 K. The molecule lies on a mirror plane in the hexagonal P /m crystal, and is close to symmetry. The C—C bonds in the tetrahedron, 1.485 (4) A, are shorter than in... 

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