Infrared and Raman spectra, conformational stability, normal coordinate analysis, barriers to internal rotations and novel DFT calculations of alicyclic 1,1-Dimethyl-1-silacyclobutane

The mid-infrared spectra (3500-400 cm^-1) of gaseous and solid 1,1-Dimethyl-1-silacyclobutane (DMSCB) have been reported. Additionally, the Raman spectra (3500-10 cm^-1) of the gas, liquid and solid phases have been recorded. Moreover, we have carried out quantum chemical (QC) calculations for (CH₂)₃Si(CH₃)₂ molecule (DMSCB) obeying C_2v/C_s point group, respectively. Initially the structures are optimized followed by frequency calculations with RHF, B3LYP and MP2 methods using 6-31G(d) and 6-31++G(d,p) basis sets. Preliminary computations favor the puckered ring conformation (C_s) by 823-1336 cm⁻¹ (2.352-3.820 kcal/mol) but an imaginary torsional frequency was obtained for configuration that obeys C_2v point group, therefore most likely excluded. The optimized structural parameters (SPs) of the puckered conformer are well correlated to those obtained from electron diffraction techniques. Aided by MP2 potential surface scans (PSS) based on the optimized structural parameters (SPs), single-point energies at 6-31++G(d,p) basis set, the (CH₃)ₐₓ/(CH₃)ₑ_q three-fold barriers of 544/506 cm⁻¹ (1.56/1.45 kcal.mol⁻¹) were estimated compared to 460/431 cm⁻¹ (1.32/1.23 kcal.mol⁻¹), employing B3LYP method. The observed infrared (IR) and Raman (R) spectra are compared with the simulated IR and Raman spectra using the estimated infrared intensities and the Raman activities, respectively from the above-mentioned methodsy. Nevertheless, frequencies were also estimated for, (CD₂)₃Si(CH₃)₂-d₆ and (CH₂)₃Si(CD₃)₂-d₆ to confirm our interpretations based on blue shifts of methylene and methyl fundamentals upon deuteration, respectively. Consequently, we have proposed a new spectral analysis for DMSCB based on C_S symmetry, which is supported by NCA, unscaled force constants (FCs) in internal coordinates and potential energy distributions (PEDs).