Abstract:
The free molecule octahydridosilasequioxane, Si8O12H8, was computationally studied, as
well as embedded in the unit cell. The point group of the free molecule is indeed Oh, and
its crystal symmetry is reduced to C3i, thus confirming the occurrence of two different
types of Si−O−Si bond lengths found experimentally. The molecular orbitals of the free
molecule show that some electron density occurs in the cubic cavity, thus contributing to
the opening of the Si−O−Si angle. A study of the packing in the unit cell identifies a new
type of packing scheme in which eight (partial) molecules participate: each apex H atom
of one protruding Si−H bond of every molecule points to the corner of an equilateral
triangle having 2.631Å sides. All hydrogen atoms in both the free molecule and in the
solid state carry negative partial charges. The reason for this is also explored, as well as
its consequences for the unique packing scheme.
