3-DIMENSIONAL HUCKEL MOLECULAR-ORBITAL ENERGY-LEVEL CORRELATION DIAGRAMS FOR POLYHEDRAL REARRANGEMENTS

A three-dimensional Huckel method recently developed for cluster compo unds has been used to investigate polyhedral rearrangements of organic molecules as well as main-group inorganic clusters. The method starts from information about atomic connectivity and number of cluster elec trons, familiar and convenient concepts for chemists. Calculations lea d to diagrams that show how molecular orbital (MO) energy levels and s econd moment scaled total energies correlate between different structu ral forms. We consider diamond-square-diamond (DSD) framework reorgani zation mechanisms for pseudo-rotations of the closo-boranes. For B5H52 - and B9H92-, is not allowed by the principle of conservation of orbit al symmetry. DSD rearrangements are allowed for the other closo-borane s we studied here, with increasing activation energies, B8H82- similar to B11H112- similar to B10H102- < B6H62-, depending on the number of square faces opened during rearrangement. MO correlation diagrams and corresponding total energy curves for various numbers of cluster elect rons provide visual rationalizations of observed structural trends wit h different numbers of cluster electrons. In particular? we discuss fr amework reorganizations for square to tetrahedron, hexagon to trigonal prism, hexagon to octahedron and cube to square antiprism.