VIBRATIONAL ANALYSIS OF INFRARED-SPECTRA OF MATRIX-ISOLATED TRANSIENTSILENES ON BASIS OF DENSITY-FUNCTIONAL THEORY CALCULATIONS

The infrared spectra of matrix-isolated transient silenes, R2Si=CR2, w ith the substituents R such as H, D, Cl, CH3, CD3 at Si and H, D, CH3 at C, have been interpreted by comparison with the density functional theory (DFT) B3LYP/6-311G(d,p), calculated harmonic vibrational freque ncies and infrared intensities and additional computation of potential energy distribution for normal modes. The calculated unsealed frequen cies deviate only by 2%-3% from the experimental ones and thus demonst rate the high reliability of the DFT approximation in predicting the s pectra of silenes, out-performing the ab initio methods in accuracy. T he DFT-based vibrational analysis confirms the previous CI/DZ + P norm al mode description given for parent silene H2Si=CH2 by Schaefer, and substantially changes the MNDO or force field approximated assignments for observed IR absorptions in all other studied silenes, in particul ar, those of the CH2 (CD2) wagging modes. These ''olefinic'' frequenci es are found to be red-shifted by 200-250 cm(-1), relative to those in corresponding alkenes. The Si=C force constants in internal coordinat es and pi-bond orders, calculated to be within 5.36-5.75 and 1.62-1.71 mdyn/Angstrom, respectively, are in line with the changes in the Si=C bond lengths depending on the substituents. (C) 1998 Elsevier Science B.V.