Characterization and Structure of Trans, Trans, Trans-1,3,5,7-Tetrakis(3,3,3-Trifluoropropyl)-1,3,5,7-Tetramet hylcyclotetrasiloxane, and Structure of Trans-1,3,5-(3,3,3-Trifluoropropyl)-1,3,5-Trimethylcyclotrisil oxane

One of four possible stereoisomers of a methyl(trifluoropropyl)siloxane cyclic tetramer was isolated from low molecular weight volatiles, formed in a reaction mixture during ring-opening/equilibration polymerization of methyl(3,3,3-trifluoropropyl)cyclotrisiloxane (D-3(F)). The crystalline solid was recognized as resembling a previously isolated cyclic siloxane with undetermined stereochemistry, assigned as a cyclotetrasiloxane by IR spectroscopy, and then further characterized by multinuclear (H-1, C-13, F-19, and Si-29) NMR spectroscopy as a relatively high symmetry species with respect to the potential cis-trans relationships of the methyl and 3,3,3-trifluoropropyl substituents at Si. Unambiguous characterization and assignment of the stereochemistry was accomplished by a single crystal X-ray diffraction study, which revealed the compound was the trans, trans, trans-isomer, one species in these equilibration polymerization mixtures. The single crystal X-ray structure of the related, commercially important trans-D-3(F) cyclotrisiloxane has also been determined. The structures of the two cyclic fluorinated siloxanes are discussed with respect to each other, and in the context of prototypical structures for these common Si3O3 and Si4O4 ring systems.