Far-Infrared Spectra and the Ring-Puckering Potential Function of Silacyclopent-3-ene and Silacyclopent-3-ene-1,1-d2

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Abstract
A series of a far‐infrared absorption bands have been observed between 35 and 100 cm−1 in the spectra of silacyclopent‐3‐ene, CH 2 CH = CHCH 2 SiH 2 , and silacyclopent‐3‐ene‐1,1‐d 2. These absorptions result from the transitions between the various energy levels of the ring‐puckering vibration. The frequencies of the bands can be calculated quite accurately using a pure quartic potential, V = 14.34Z 4 for the hydride and V = 13.20Z 4 for the deuteride where Z is the ring‐puckering coordinate in reduced form. A slightly better fit for each spectrum is obtained by adding a small positive quadratic term to the potential function. The ring‐puckering potential for this molecule shows that the structure of the five‐membered ring is a planar one and that it has no barrier to inversion like that in cyclopentene. This lack of a potential barrier is attributed to a smaller barrier to internal rotation in the –CH2–SiH2– bond as compared to –CH2–CH2–. Also there is evidence from the isotope shift of the frequencies (which is related to the reduced masses of the 2 isotopic species) that the distance between the silicon atom and the double bond is much smaller than expected. This suggests that a weak bond between the Si d orbitals and the π system is partly responsible for the planarity of the compound.

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