The two-dimensional potential energy surface for the ring puckering and ring twisting of 1-silacyclopent-3-ene-d, 1-d1, and 1, 1-d2

The far-infrared spectra of 1-silacyclopent-3-ene and an isotopic mixture containing both 1-d1 and 1, 1-d2 species have been recorded and analyzed. In addition to the main ring-puckering series, a weak side band series, resulting from molecules in the ring-twisting excited state, was observed for the undeuterated molecule. In the 330–520 cm−1 region of the infrared spectrum of the isotopic mixture, combination bands of the puckering with the SiHD or SiD2 rocking were observed. The ring-twisting fundamental was also assigned to the 380.6 cm−1 band in this region. Analysis of the data for all three isotopic species results in the following two-dimensional potential energy surface: V(cm−1)=2.08×105x41−0.005×105x21 +0.547×105x22+1.70×105x21x22 where x1 and x2 are the ring-puckering and ring-twisting coordinates (in Å), respectively. The potential minimum corresponds to a planar conformation and the positive cross term shows the vibrational interaction to be anticooperative.