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.
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