Computational study of the structural and vibrational properties of ten and twelve vertex closo-carboranes

Abstract

Calculations using ab initio Hartree-Fock and Density Functional theories, the latter employing the B3LYP functional, in combination with a number of large standard basis sets ranging from 6-31G** to cc-pVDZ, have been performed on a series of ten and twelve vertex closo-carborane isomer species. Results obtained for optimized structural parameters and molecular properties are presented for 1,2-, 1,6- and 1,10-C2B8H10 and 1,2-, 1,7- and 1,12-C2B10H12 and compared, where possible, with both earlier theoretical data and experiment. Irrespective of the model chemistry chosen, the para-isomer in each class of carborane cluster is found to be the most stable species, corresponding to a structure in which the cage carbon atoms are positioned at diametrically opposed ends of the respective polyhedron. Boron-hydrogen and carbon-hydrogen bond lengths are found to change little on going from isomers of one particular cage size to another, supporting analogous conclusions previously established for small closo-carborane cages possessing five, six and seven vertices. The calculated vibrational spectra of the isomers of both decacarborane and do-decacarborane are seen to be similar to each other and reflect a high degree of rigidity within each cluster. Key polyhedral skeletal breathing modes along with characteristic boron-hydrogen and carbon-hydrogen stretching frequencies are identified in the spectra and compared with experiment. Thermochernical data relating to each species are also analyzed. (C) 2002 Elsevier Science B.V. All rights reserved.