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 dodecacarborane 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. Thermochemical data relating to each species are also analyzed. 2002 Elsevier Science B.V. All rights reserved.