Further insights on the molecular structure, vibrational spectra and ring-puckering potentials of silacyclopent-3-ene and its 1,1-dihalo derivatives: Ab initio and DFT study

The structure, ring-puckering potential energy functions and vibrational spectra of silacyclopent-3-ene (3SCP) and its 1,1-d ₂ (3SCP-d ₂), 1,1-difluoro (3SCP-F ₂), 1,1-dichloro (3SCP-Cl ₂) and 1,1-dibromo (3SCP-Br ₂) derivatives were investigated using ab initio MP2 and MP4(SDQ), and BLYP, B3LYP and BB1K density functional calculations. The stable form of 3SCP was obtained from MP4(SDQ) and BB1K levels to be unpuckered with a modest nonplanarity character in excellent agreement with previous experimental results. The 3SCP-F ₂ ring was predicted by all levels of theory to be the most rigid in the series, while the dichloro and dibromo analogs were predicted to maintain a slightly puckered structure with very small barrier height in terms of the puckering motion. When compared with the results obtained from the MP4(SDQ) theory with the 6-311G(d,p), 6-311+G(d,p) and 6-311++G(d,p) basis sets, the BB1K functional showed noticeable performance over B3LYP and MP2 methods in predicting reliable ring-puckering angle and barrier for all members of the 3SCP series. The infrared and Raman spectra of the investigated cyclic organosilanes were revisited on the basis of the frequencies calculated at B3LYP and BB1K levels and the derived potential energy distribution values.