CALCULATIONS FOR VARIOUS STRUCTURAL FORMS OF B12H122- AS CLUES TO THE POSSIBLE MECHANISMS FOR THE ISOMERIZATIONS OF C2B10H12

We report the results of geometry-optimized AM1 and ab initio SCF-MO calculations at the STO-3G level for B12H12(2-) in several structural forms that represent intermediates in various mechanisms proposed for the isomerization of the carboranes o-, m-, and p-C2B10H12. We assume that the energies of the B12H12(2-) structures, relative to that of the regular icosahedron, represent estimates of activation barriers that the corresponding carboranes might meet along the proposed mechanisms. Structures studied were the regular icosahedron (I(h)), an icosahedron of D3h symmetry, the cuboctahedron (O(h)), the bicapped pentagonal prism (D5h), the anticuboctahedron (D3h), the tetracapped cube (D4h), the truncated tetrahedron (T(d)), and a hexagonal antiprism (D6d). Structural parameters and total energies of these structures are tabulated. Each mechanism can be represented by a reaction graph, the connectivity of which describes the pattern of isomeric conversions for C2B10H12. An acceptable mechanism must have a low activation barrier and require minimal atomic motions, and its reaction graph must account for the experimental observations of isomerizations. The lowest energy pathway seems to be one involving triangular-face rotation. Next higher in energy is the mechanism that passes through the cuboctahedral structure. Carborane isomerization reactions pass through specific transition-state isomers. Lacking calculated energies for these isomers, one can estimate their relative stabilities qualitatively by empirical valence rules and the rule of topological charge stabilization. These considerations give insight into the details of individual processes and, together with the calculated energy results for B12H12(2-), provide new support for triangular-face rotation as the mechanism that can best account for the observed isomerizations of o-, m-, and p-C2B10H12.