  The fully halogenated or hydrogenated B12X12 (X = H, F, Cl, Br and I)
clusters are confirmed to be icosahedral. On the other hand, the bare B12
cluster is shown to have a planar structure. A previous study showed that a
transformation from an icosahedron to a plane happens when 5 to 7 iodine atoms
are substituted. Later, the transition was confirmed to be seven iodine
substitutions based on an infrared spectroscopy study. In this study, we
investigated the effects of different halogen atoms on the opening of the B12
icosahedral cage by means of density functional theory calculations. We found
that the halogen elements do not have significant effects on the geometries of
the clusters. The computed IR spectra show similar representative peaks for all
halogen substituted clusters. Interestingly, we found a blue-shift in the IR
spectra with the increase in the mass of the halogen atoms. Further, we
compared the Gibbs free energies at different temperatures for different
halogen atoms. The results show that the Gibbs free energy differences between
open and close structures of B12X7 become larger when heavier halogen atoms are
present. This interesting finding was subsequently investigated by energy
decomposition analysis.
