Disorder and pseudo-symmetry in octakis(trivinylsilyl)octasilicate

Octakis(trivinylsilyl)octasilicate was prepared by capping octaspherosilicate cubes, [Si(8)O(20)](8-), with trivinylsilyl groups in methanol solution. Crystals grown from CCl(4) crystallize in the tetragonal space group I4(1). Systematic absences are consistent with the space group I4(1)/amd, although the R(int) values clearly indicate 4/m rather than 4/mmm Laue symmetry. Structure solution and refinement show that the pseudo a-glide results from the approximate m\bar 3m symmetry of the core (Si(8)O(12))O(8)(8-) unit. The positions of the molecules conform to a {110} d-glide that is broken by the small rotations of all the molecules in the same direction about [001]. Crystals grown from toluene give a diffraction pattern consisting of sharp peaks that can be indexed on the same ca 7200 A(3) unit cell, but with h + k even, and l even only. The l = odd layers contain no Bragg spots, but instead exhibit diffuse sheets of intensity. Within the sheets of diffuse scattering are streaks parallel to r* = 110* that cross at the h + k odd Bragg positions. This diffuse scattering pattern arises from well ordered rods of molecules parallel to c with frequent faults in the stacking sequence of molecules parallel to 110, with displacement vectors of [00(1/2)].