Ab initio and density functional theory calculation of the structure and vibrational properties of n-vertex closo-carboranes, n=5, 6 and 7

An extensive high-level computational study of various possible isomers of five, six and seven vertex closo -carboranes has been carried out using ab initio self-consistent field and density functional (DFT) theories. In total nine different cage structures have been investigated at the Hartree–Fock and B3LYP DFT levels in conjunction with the 6-31G** and 6-31++G** basis sets. Energies, optimized geometries, Mulliken charges, harmonic frequencies and electric dipole and quadrupole moments have been computed and compared with previous calculations and experimental data where applicable. The inclusion of diffuse functions in the basis set is shown not to significantly affect the results obtained for structural parameters and molecular properties. For a fixed cluster size, and for whichever model chemistry is chosen, the energetically most stable isomer is computed to be 1,5-C 2 B 3 H 5 , 1,6-C 2 B 4 H 6 and 2,4-C 2 B 5 H 7 . It is also found that irrespective of cluster vertex number, boron–hydrogen and carbon–hydrogen bond lengths remain relatively constant. The simulated vibrational spectra are analysed in detail and characteristic boron–hydrogen and carbon–hydrogen stretching frequencies and skeletal breathing modes are identified.