A computational study of the molecular and crystal structure and selected physical properties of octahydridosilasequioxane—(Si₂O₃H₂)₄. I. Electronic and structural aspects

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 O h , and its crystal symmetry is reduced to C 3i , 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 2.631 Å 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.