3-DIMENSIONAL HUCKEL THEORY FOR CLOSO-CARBORANES

We have;recently developed a 3-dimensional Huckel method for cluster c ompounds. The method uses a set of approximations for Coulomb, resonan ce, and overlap integrals very similar to those employed in the famili ar 2-dimensional Huckel theory for the pi electrons of planar conjugat ed hydrocarbons. The method can be adapted to heteroatomic clusters by introducing heteroatomic Coulomb integrals, alpha(Y) = alpha(X) + h b eta, where it is a parameter for heteroatom Y. In this paper, we use t he 3-dimensional Huckel method to study the properties of the closo-ca rboranes, C2Bn-2Hn. We calibrate the method by choosing a value of the heteroatomic parameter it that distinguishes positional isomers by en ergy and gives them relative energies in rough agreement with those es tablished by observation and ab initio calculations. We obtain modest improvement in matching ab initio relative energies of isomers by mean s of a three-parameter, first-order perturbation treatment. We use the calibrated method to evaluate various mechanisms proposed for the iso merizations of C2B4H6, C2B5H7, and C2B6H8, all of which have been obse rved to undergo intramolecular isomerizations. Rearrangements of C2B6H 8 have been satisfactorily explained by a single-DSD (diamond-square-d iamond) process. Those for C2B5H7 require at least two DSD processes, concerted, consecutive, or overlapping. Several different mechanisms h ave been proposed for the rearrangement of C2B4H6. In evaluating inter mediate and transition State structures, the 3-dimensional Huckel meth od gives higher energies to those structures with a larger number of n ontriangular faces, a plausible conclusion except that occasionally it is wrong. In comparison with ab initio results, the 3-dimensional Huc kel method fails to give low energies for classical structures.