Tetrahedranes strain energy

FIGURE 1. The strain energies of the tetrahedrane and [n]prismane system (M2 H2 ) calculated at the HF/6-31G level for M = C and Si and the HF/DZ(d) level for M = Ge and Sn. Reprinted with permission from Reference 10. Copyright 1995 American Chemical Society... 

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. 

Thermochemical data substantiated the highly strained character of 4. In fact, the strain energy per C-C bond of the tetrahedrane skeleton in 4 turned out to be 21.5 kcal mol , the highest value known (c/ data for cubane, vide supra). Why then is it so stable Obviously this is a purely kinetic stability, as was the case with cubane, but the origin of this stability of 4 is rather unusual. It is speculated that the presence of four terf-butyl groups creates a kind of corset ... 

Table I summarizes the calculated strain energies of compound of the type E H , where n is 4, 6, and 8 and E is C, Si, Ge, and Pb, derived from homodesmic reactions (16b). The strain energies of silicon and carbon tetrahedranes are very similar (140 kcal/mol). However, replacement of the three-membered rings by four-membered rings results in a significant decrease in strain energy 140.3 (Si4H4) > 118.2 (SieHe) > 99.1 (SigHg)...

Figure 3-28. Polyhedrane molecules, (a) Tetrahedrane, (CH). It has very high strain energy and has not (yet ) been prepared, (b) Tetra-tert-butyltetrahedrane, CiClCHjlj], [3-31). (c) Cubane, (CH)g, [3-32]. (d) Dodecahedrane, (CH)jj, [3-11]. (e) [3-34], not yet prepared.

The use of isodesmic-type reactions (method (b)) to calculate the strain energy of tetrahedrane is shown (calculations by the author) it is instractive to examine... 

The compact cage molecules, octanitrocubane and tetranitro tetraazacubane and diazadinitro tetrahedrane, have molecular strain energy locked-in which is released along with the internal combustion energy when they explode. The strain energies in these molecules are high as demonstrated in Table 4 for the case of cubane. 

Exotic chemistry the concatenation of strained rings in tetrahedrane may not ordy thwart conventional routes to its realization, but could also confer unusual properties, such as anomalous reactivity, acidity or basicity, and ionization energy, to name some possibilities. 

---