Vibrational analysis of infrared spectra of matrix-isolated transient silenes on basis of density functional theory calculations

Abstract

The infrared spectra of matrix-isolated transient silenes, R 2SiC R 2, with the substituents R such as H, D, Cl, CH 3, CD 3 at Si and H, D, CH 3 at C, have been interpreted by comparison with the density functional theory (DFT) B3LYP/6-311G(d,p), calculated harmonic vibrational frequencies and infrared intensities and additional computation of potential energy distribution for normal modes. The calculated unscaled frequencies deviate only by 2%–3% from the experimental ones and thus demonstrate the high reliability of the DFT approximation in predicting the spectra of silenes, out-performing the ab initio methods in accuracy. The DFT-based vibrational analysis confirms the previous CI/DZ + P normal mode description given for parent silene H 2SiCH 2 by Schaefer, and substantially changes the MNDO or force field approximated assignments for observed IR absorptions in all other studied silenes, in particular, those of the CH 2 (CD 2) wagging modes. These “olefinic” frequencies are found to be red-shifted by 200–250 cm −1, relative to those in corresponding alkenes. The SiC force constants in internal coordinates and π-bond orders, calculated to be within 5.36–5.75 and 1.62–1.71 mdyn/Å, respectively, are in line with the changes in the SiC bond lengths depending on the substituents.