The gas-phase structure of octaphenyloctasilsesquioxane Si8O12Ph8 and the crystal structures of Si8O12(p-tolyl)8 and Si8O12(p-ClCH2C6H4)8

Abstract

The equilibrium molecular structure of octaphenyloctasilsesquioxane Si₈O₁₂Ph₈ in the gas phase has been determined by electron diffraction. It was found to have D₄ point-group symmetry, with Si–O bond lengths of 1.634(15)–1.645(19) Å, and a narrow range [147.5(45)–149.8(24)°] of Si–O–Si angles. The structures of Si₈O₁₂(p-tolyl)₈ and Si₈O₁₂(p-ClCH₂C₆H₄)₈ have been determined by X-ray diffraction and are found to have Si₈O₁₂ cages significantly distorted from the symmetry found for Si₈O₁₂Ph₈ in the gas phase. Thus, Si–O–Si angles range between 144.2(2)–151.64(16)° for Si₈O₁₂(p-tolyl)₈, and between 138.8(2)–164.2(2)° for Si₈O₁₂(p-ClCH₂C₆H₄)₈. These three structures show how much a Si₈O₁₂ cage may be distorted away from an ideal structure, free from intermolecular forces, by packing forces in a crystalline lattice.