Theoretical Calculations, Microwave Spectroscopy, and Ring-Puckering Vibrations of 1,1-Dihalosilacyclopent-2-enes.

High-level theoretical CCSD/cc-pVTZ computations have been carried out to calculate the structures and ring-puckering potential energy functions (PEFs) for 1,1-difluorosilacyclopent-2-ene (2SiCPF) and 1,1-dichlorosilacyclopent-2-ene (2SiCPCl). The structure and PEF for 1,1-dibromosilacyclopent-2-ene (2SiCPBr) were obtained by ab initio MP2/cc-pVTZ computations. The parent silacyclopent-2-ene (2SiCP) is puckered with a 49 cm barrier to planarity, 2SiCPF has a planar ring system, 2SiCPCl has a calculated tiny 4 cm barrier but is essentially planar, and the dibromide has a calculated barrier of 36 cm. Microwave spectra of seven isotopic species of 2SiCPF were recorded on a chirped pulse, Fourier transform microwave (CP-FTMW) spectrometer in the 6-18 GHz region. The -type and -type transitions were observed. The rotational constants and three quartic centrifugal distortion constants were determined for the parent, Si, Si, and all singly substituted C isotopologues in natural abundance. This allowed for the determination of the heavy-atom structure of the ring and showed the ring to be planar. The experimentally determined rotational constants and geometrical parameters agree very well with the theoretical values and confirm the planarity of the five-membered ring. A comparison of the PEFs for the silane and the three dihalides shows the silane to have the stiffest puckering motion and the dibromide to be the least rigid.