The molecular structure of cyclic methylsiloxanes

A series of methylcyclosiloxanes from the trimer to the hexamer has been investigated by gas phase electron diffraction. The Si-O bond distance varies between 1.635 and 1.620 Å, the SiOSi angle has values from 132° to 149° within the series. The trimer is substantially planar, the best agreement for the tetramer was obtained with a model of S₄. symmetry. As a consequence of large amplitude vibrations, the larger rings do not possess a well-defined conformation. The results are discussed in relation to previous spectroscopic investigations.