Far‐Infrared Spectrum and the Barrier to Pseudorotation of Silacyclopentane
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The far-infrared spectrum arising from the transitions between the pseudorotational levels in silacyclopentane has been observed. Thirteen absorption maxima were found for the v=0 (radial ground) state and eight for the v= 1 state. For each series a potential of the form V= (V2/2) (1+cos2c£) predicts frequencies which agree very closely with the observed values. The values of V2, which represent the barriers to pseudorotation, were found to be 1362±25 cm-1 for the radial ground state and 1301±50 cm-1 for the first excited state. The pseudorotation constants for the two radial states were found to be B 0= 1.966 and B1 = 2.033 cm-1. The observed pseudorotation barrier of 3.89 kcal/mole represents the energy required to go from the more stable C2 half-chair conformation to the C3 envelope form and is higher than expected from previously derived formulas.
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