RING-PUCKERING POTENTIAL-ENERGY FUNCTION AND UNUSUAL RIGIDITY OF SILACYCLOPENT-2-ENE
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The far-infrared spectrum resulting from the ring-puckering vibration of silacyclopent-2-ene-1,1-d2 has been recorded, and several transitions have been observed between 100 and 200 cm-1. The data confirm that the ring system is planar. The reduced masses for both undeuteriated and d2 molecules have been calculated as a function of an SiH2 rocking parameter. When a small amount of rocking is mixed with the puckering motion, the same one-dimensional potential energy function can be used to fit the far-infrared spectra of both isotopic species. The function derived is V = 20.75 105x4 + 19.59 103x2, where x is the puckering coordinate. The large potential constants imply that an extremely rigid ring system is present. Molecular mechanics calculations based on standard force constants, which result in satisfactory predictions for related molecules, predict the ring should be nonplanar and much less rigid. Modifications of torsional and angle bending force constants, which are expected to be increased in magnitude by silicon- orbital interactions, yield a much better agreement between the molecular mechanics and experimental potential energy function. 1988 American Chemical Society.
