Knowledge Bank

The far-infrared spectrum of silacyclopent-2-ene, $CH=CHSiH2CH2CH2$, was recorded and a series of weak absorption peaks were observed between $120$ and $200cm-1$. The absorptions arise from the ring-puckering vibration and the frequencies can be fit well with a potential of the form $V=Ax4+Bx2$ where x is the puckering coordinate and A and B are both positive constants. The results show that the molecule is planar and unusually rigid in this position. The sulfur and oxygen analogs of this ring compound are puckered molecules; the unexpected planarity of the silane may be due to $p\pi -d\pi$ bonds involving the silicon atom and the double bond.