No abstract

No abstract

THE STRUCTURES OF DISILOXANE AND HEXAMETHYLDISILOXANE equipment and microdensitometer intensities were obtained from films exposed using Cu Ka radiation. The structures were refined to R = 4-2% over 403 reflexions for disiloxane, and R = 3.5% over 1256 reflexions for hexamethyldisiloxane. Crystalline disiloxane has distorted mm2 molecular symmetry with Si-O = 1.631 (6) A and Si-O-Si = 142.2 (3) °. Crystalline hexamethyldisiloxane has distorted m molecular symmetry with Si-O = 1.626 (5) A and Si-O-Si = 148.8 (2) °. Molecules of disiloxane form infinite chains as a result of weak, but highly directional, intermolecular Si..-O interactions (3.115 A). Hexamethyldisiloxane exists as discrete molecules with no short intermolecular contacts. IntroductionIn water, dimethyl ether, F20 and C120 , the angle at oxygen is close to or less than the tetrahedral value. In silicates and disiloxanes, the angle at oxygen is very much wider. Moreover, it increases from the apparent value of 144.1 ° in disiloxane itself (as measured by electron diffraction in the gas phase) to 180 ° in crystalline hexaphenyldisiloxane. Values for Si-O-Si and Si-O distances in a number of disiloxanes and related compounds are given in Table 7.Most of the features in the vibrational spectra of disiloxane can be interpreted in terms of a model with a linear heavy-atom skeleton (Lord, Robinson & Schumb, 1956). This apparently conflicts with the structure as determined by electron diffraction (Almenningen, Bastiansen, Ewing, Hedberg & Traetteberg, 1963); the two can be reconciled if the molecule is regarded as 'quasi-linear': bent but with a skeletal bending vibration that is low in frequency and large in amplitude. Under these conditions the language of the conventional point-group analysis becomes inappropriate; permutation group theory must be used instead. The appropriate permutation group is isomorphous with D3h , leading to selection rules that are effectively the same as for a molecule with a linear heavy-atom skeleton. A detailed analysis of the weak Raman feature corresponding to the overtone of the skeletal bending mode has led Durig, Flanagan & Kalasinsky (1977) to propose a potential surface with a hump at a linear configuration of height 112 cm -1, and a minimum corresponding to a Si-O-Si angle of 149 °.The concept of quasi-linearity is by now well established, but little is known about the structures of quasilinear molecules in the solid state. The vibrational spectra of disiloxane and hexachlorodisiloxane change on crystallization, implying that there might be associated changes in structure. Disiloxane is a clearly recognized example of quasi-linearity, while it is suggested that features in the Raman spectra of hexamethyldisiloxane imply that this molecule too should be regarded as quasi-linear. We here present the crystal structures of disiloxane and of hexamethyldisiloxane. ExperimentalCrystallographic data ProcedurePure samples of disiloxane and hexamethyldisiloxane were sealed into glass capillaries, which were mounted on Tufnol ins...

No abstract

No abstract