Abstract
The Raman spectrum (4000–200 cm^−1^) of liquid and infrared spectra (4000–400 cm^−1^) of gaseous and liquid 2,2,5,5‐tetramethyl‐2,5‐disila‐1‐oxacyclopentane, c‐OSi(CH~3~)~2~CH~2~CH~2~Si(CH~3~)~2~, have been recorded. Ab initio and density functional theory (DFT) calculations with several different basis sets including diffuse functions have been carried out to predict the conformational stabilities with comparisons to disiloxane (H~3~SiOSiH~3~)and 2,5‐disila‐1‐oxacyclopentane (c‐OSiH~2~CH~2~CH~2~SiH~2~). These calculations predict the twisted C~2~ conformer as the most stable form of the ring compounds, with the planar C~2__v__~ conformer a transition state and the envelope C~s~ conformer not a stable form. These predicted stabilities are supported by the vibrational data. Additionally, force constants, infrared intensities, Raman activities, depolarization ratios and scaled vibrational wavenumbers have been determined from MP2(full)/6–31G(d) calculations for all three molecules and compared with experimental values when available. Predicted r~0~ structural parameters have been obtained for dimethyl ether, methyl silyl ether and disiloxane, which are compared with the experimentally determined values. On the basis of these data, estimated r~0~ parameters are provided for both five‐membered rings with the expectation that these values are as accurate as could be measured experimentally in the gas phase. The results are discussed and compared with corresponding quantities for some similar molecules. Copyright © 2006 John Wiley & Sons, Ltd.