The crystal structure of octamethylspiro [5.5] pentasiloxane: rotation about the ionic silicon-oxygen bond

The crystal structure of octamethylspiro [5·5] pentasiloxane is reported. The Si-O distance is 1.64 ± 0.03 A. and the Si-CH₃ distance 1.88 ± 0.03 A. The (SiO)₃ ring is planar and the shape is determined by the tetrahedral silicon angles. Evidence is presented for the libration of methyl groups about siloxy bonds, and this is interpreted in terms of the ionic character of the Si-O bond. It is suggested that free rotation of CH₃ groups about siloxy bonds in the linear polymers is responsible for the small intermolecular forces characteristic of these compounds.