Ab initio STO-3G molecular orbital theory has been used to calculate energy-optimized geometries for cyclotrisiloxane and cyclotetrasiloxane. The resulting SiOSi angles are in close correspondence with experimental SiOSi angle frequency distributions for tetrahedral rings in silicates and siloxanes. Experimental correlations between SiO bond length and bridging SiOSi angle for rings in silicates and siloxanes are reproduced by the theory. The stable configurations of 4-membered tetrahedral rings are predicted to have the exact reverse sequence to that indicated by previous electrostatic potential energy calculations. The observed configurations of the tetrahedral sheets in gillespite and apophyllite conform with the calculated stable configurations of 4-membered rings with Cnv point symmetry. Ascribing the configuration of the silicate sheets to the constitution of the interlayer ions does not appear to be necessary. CNDO/2 calculations for Dnh and Cnv hydroxycyclosiloxane molecules were used to compare the relative stability of different size silicate rings. The 3-membered ring is indicated to be unstable relative to larger rings due to strained SiOSi angles ~130°, consistent with its breakdown during a trimethylsilylation reaction. Its presence in silicate glasses and melts must also be infrequent. Larger rings assume nonplanar configurations to achieve minimum-energy SiOSi angles averaging 147°.

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