Schwingungsspektren und Normalkoordinatenanalyse der isotopomeren closo-Hexaiodohexaborate 11Bn10B6-nI62-, n = 0 - 6
Normal coordinate analyses for the ten isotopomers Cs211Bn10B6-nI6, n = 0 - 6 , including the pairs of geometric isomers for n = 2, 3, 4 have been performed, based on a general valence force field. Using a set o f 8 force constants (e.g. fd(BB) = 1.45, fd(BI) = 2.33 mdyn/Å ) the calculated frequencies are in good agreement with observed IR and R am an bands measured at 10 or 80 K for Cs2B6I6 with natural abundance of B isotopes (~80% 11B , 20% 10B) as well as for 11B and 10B enriched derivatives. From the calculated atomic displacements and the potential energy distribution the classification in the 30 times more intense substituent (in-phase motions of B and I, ≤ 153 cm-1) and the weak cage vibrations (anti-phase displacements of B and I, ≥ 385 cm-1) is deduced. Only the cage modes show characteristic band patterns, explained by the superposition of the calculated spectra of the different isotopomers in accordance with their statistical abundance. There is an excellent conformity for the total symmetric A, vibration, but for the triply degenerate T lu and T 2g modes as a consequence of lowered site symmetry in the solid state more bands than calculated are observed.