The equilibrium molecular structure of the decasilsesquioxane, Si 10 O 15 H 10 , in the gas phase has been determined by gas electron diffraction. Molecular dynamics calculations were used to give amplitudes of vibration and differences between interatomic distances in the equilibrium structure and the vibrationally averaged distances that are given directly by the diffraction data. The molecules have D 5h symmetry, and do not show the distortions that are apparent in the crystalline phase. The

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... ten-membered silicon-oxygen rings are found to be particularly flexible in the gas phase, a phenomenon that was also seen in crystal structures. The Si-O bond lengths in the ten-membered rings are 161.6(2) pm long and in the eight-membered rings they are 162.2(3) pm, with Si-O-Si angles of 155.0(5) and 153.9(7) • , respectively.