Far-infrared spectra and the ring-puckering potential function of silacyclopent-3-ene and silacyclopent-3-ene-1,l-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 2CH=CHCH2SiH2, and silacyclopent-3-ene-1,1- d2. 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=13.20Z4 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 -CH 2-SiH2- bond as compared to -CH2-CH 2-. 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 system is partly responsible for the planarity of the compound.
