Dodecahedrane strain energy

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 high strain energy of cubane results in a very high proton affinity, and low-level theory predicts that protonation occurs at a corner . The high basicity of the dodecahedranes cannot be entirely due to relief of strain and is attributed to the high polarizability of this molecule . 

Each Ti bonds to 3C via a bonds and each C bonds to 2Ti and one C. The all-carbon analogue, C20, is not expected to be stable because of severe internal strain (it would be the smallest possible fullerene, p. 280). Note, however, that dodecahedrane, C20H20, is known.An alternative description of the structure (Fig. 8.18b) would be as a weakly bonded cube. Tig, each face of which is capped by a C2 unit. The calculated distances are Ti Ti 302 pm, Ti-C 199 pm and C-C 140 pm (implying some multiple bonding cf. 140 pm in benzene). An alternative Th structure for TigC, which is calculated to have a lower energy, has also been proposed.In this, the Tig array is a tetracapped tetrahedron containing six Ti4 faces in butterfly conformation each of these Ti4 faces can then accommodate a C2 unit as shown in Fig. 8.18c. 

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