A conformational analysis of eight-membered cyclosiloxanes

UDC 541.636 The method of atom--atom potential functions has been used to study the conformational possibilities of the tetrasiloxane ring, present in various organosilicon monomers and polymers. In the first part, the regions of conformational space satisfying the condition for ring closure are determined, and, in the second part, the relative energies of the canonical symmetrical forms are calculated, and the range of existence of the low-energy conformation established. It has been shown that the tetrasiloxane ring is much more flexible than cyclooctane: The typical energy difference between its conformations is 0.1-0.2 kcal/mole. The small difference in the conformational energies is related to the large variety of unsymmetrical structures of the tetrasiloxane ring observed in crystals, which can be represented approximately as the superposition of the symmetrical canonical forms.