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 $\text{υ\hspace{0.17em}=\hspace{0.17em}0}$ (radial ground) state and eight for the $\text{υ\hspace{0.17em}=\hspace{0.17em}1}$ state. For each series a potential of the form ${\mathrm{V\hspace{0.17em}=\hspace{0.17em}(V}}_2\text{\hspace{0.17em}/\hspace{0.17em}2) (1\hspace{0.17em}+\hspace{0.17em}}\cos \text{2φ)}$ predicts frequencies which agree very closely with the observed values. The values of $V_2$ , 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 $B_0\text{\hspace{0.17em}=\hspace{0.17em}1.966}$ and $B_1\text{\hspace{0.17em}=\hspace{0.17em}2.033}{\mathrm{cm}}^{\text{−1}}$ . 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 ${\mathrm{C}}_s$ envelope form and is higher than expected from previously derived formulas.