STERIC OVERCROWDING IN PERHALOGENATED CYCLOHEXANES, DODECAHEDRANES, AND [60]FULLERANES

Results of ab initio electronic structure calculations on ethane, cyclohexane, dodecahedrane, [60]fullerane, and their perhalogenated analogs, i.e. molecules with the compositions C(2)X(6), C(6)X(12), C(20)X(20), and C(60)X(60), where X = H, F, Cl, and Br, are presented. Analysis of energies, geometries, and electron densities computed at the Hartree-Fock level of theory with basis sets of the TZP quality indicates that steric overcrowding in these molecules ranges from minor to severe. The destabilization due to steric overcrowding, assessed with the help of isodesmic transhalogenation reactions, is particularly large in the C(20)X(20) and C(60)X(60) species, -putting the recent claims of the synthesis of C20Br20 in some doubt. No energy minimum is found for the hypothetical C60Br60 system. In molecules with severe steric repulsions attractor interaction lines that connect nonbonded halogen atoms were found. In the C6Cl12 and C6Br12 species, these lines extend between axial halogens in accordance with the commonly accepted notions of organic chemistry but in variance with the expectations based on the distance between spherically symmetrical atoms, thus confirming the usefulness of the attractor interaction lines as indicators of steric overcrowding.