On the Lack of Conformational Preferences for Cyclic Dimethylsiloxane Oligomers.

Abstract

Cyclic dimethylsiloxane oligomers up to the hexadecamer have been isolated. Entropies and enthalpies of fusion, as well as infrared spectra, indicate that no conformational preferences exist in the liquid of the higher members. Only the trimer and the tetramer seem to have one well-defined conformation. The higher members (except the octamer) may have several conformations coexisting even in the crystal lattice. Trivial steric requirements of the methyl groups and the obtainment of high symmetry seem to dictate the selection of crystal conformation. Dieleetrio measurements do not permit calculation of dipole moments, as the high atomic polarization Ieads to molecular deformation in the electric field. Alternation in basicity along the series has been observed.