Far-Infrared Spectrum and Ring-Puckering Potential of Silacyclopent-2-ene

Abstract

The far‐infrared spectrum of silacyclopent‐2‐ene, $${\mathrm{CH\leftarrow CHSiH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2,$$ has been recorded, and a series of weak absroption peaks was observed between 120 and 200 cm⁻¹. The absorptions arise from the ring‐puckering vibration, and the frequencies fit well with the potential ${\text{V\hspace{0.17em}=\hspace{0.17em}31.45 (Z}}^4{\text{\hspace{0.17em}+\hspace{0.17em}2.40Z}}^2){\mathrm{cm}}^{\text{−1}}$ , where $Z$ is the puckering coordinate in reduced form. In terms of the dimensioned coordinate $X$ , the potential becomes ${\text{V\hspace{0.17em}=\hspace{0.17em}18.51\hspace{0.17em}×\hspace{0.17em}10}}^5{X}^4{\text{\hspace{0.17em}+\hspace{0.17em}18.27\hspace{0.17em}×\hspace{0.17em}10}}^3{X}^2$ . The coefficients of the terms in the potential function are unexpectedly large and show that silacyclopent‐2‐ene is planar and unusually rigid in that position. Interaction between the silicon $d$ orbitals and the carbon–carbon $\pi$ bond most likely accounts for the observed results.