Conformational stability, ab initio calculations and vibrational assignment for 1,1-difluoro- and 1,1-dichloro-1-silacyclopentane

The infrared spectrum (3200–400 cm−1) of the gas and the Raman spectrum (3200–50 cm−1) of liquid and solid 1,1-difluoro-1-silacyclopentane and 1,1-dichloro-1-silacyclopentane have been recorded. In all of these physical states only the twisted (C2) conformer was detected. The conformational energies have been calculated with the Moller–Plesset perturbation method to the second order; MP2(Full) as well as the density functional theory by the B3LYP method utilizing a variety of basis sets up to 6-311 + G(2df,2pd). All the calculations predict only the twisted form as the stable conformer of 1,1-difluoro-1-silacyclopentane with the average barrier to planarity of 2548 cm−1 (30.49 kJ/mol) from the MP2 calculations and a significantly lower value of 1422 cm−1 (17.01 kJ/mol) from the density functional calculations. Neither calculation was significantly affected by the inclusion of diffuse functions. The Cs conformer has a lower energy of 1703 cm−1 (MP2) and 1334 cm−1 (B3LYP) than the planar form. Thus the path between the two identical C2 conformers is by pseudorotational motion rather than through the planar form. Similar results obtain for 1,1-dichloro-1-silacyclopentane. The optimized geometry calculated with the 6-311 + G(2df,2pd) basis is given together with a complete vibrational assignment for the twisted (C2) conformer. These assignments are supported by normal coordinate calculations with scaled force constants from MP2(Full)/6-31G(d) calculations. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for silacyclopentane.