Far-Infrared Spectrum and the Barrier to Pseudorotation of Silacyclopentane

The far-infrared spectrum arising from the transitions between the pseudorotational levels in silacyclopentane has been observed. Thirteen absorption maxima were found for the υ = 0 (radial ground) state and eight for the υ = 1 state. For each series a potential of the form V = (V2 / 2) (1 + cos2φ) 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 B0 = 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 Cs envelope form and is higher than expected from previously derived formulas.