The ring-puckering potential energy function and theoretical calculations for silacyclopent-2-ene-d₀ and 1,1-d₂ and the difluoro and dichloro derivatives

High level ab initio and DFT calculations have been carried out for silacyclopent-2-ene and its 1,1-d₂, 1,1-difluoro, and 1,1-dichloro derivatives. The previously published far-infrared spectra of the ring-puckering vibration, which had been interpreted to be characteristic of a rigid planar molecule, have been reanalyzed for the hydride and 1,1-d₂ derivative. Both the spectra and the theoretical calculations show the molecule to have a small barrier to planarity. The experimental data analyzed with a Gaussian barrier produce a barrier of 49 cm^-1 as compared to a value of 47 cm^-1 computed using the CCSD/6-311++G(d,p) basis set. The experimental value for the deuteride was determined to be 41 cm^-1 from the one-dimensional approximation. All MP2 and DFT computations for the 1,1-difluoro derivative predict a planar structure whereas the MP2 computation when used with triple-ζ basis set predicts a barrier of 13 cm^-1 for the chloride. Vibrational frequencies were also computed for these molecules and compared to experimental results for the characteristic frequencies for these types of molecules.