Spectra and structure of silicon containing compounds. XLI. Infrared and Raman spectra, conformational stability, vibrational assignment and ab initio calculations of cyclopropylbromosilane

Abstract

The infrared (3100–40 cm −1) spectra of gaseous and solid and Raman (3200–100 cm −1) spectra of liquid cyclopropylbromosilane, c-C 3H 5SiH 2Br, have been recorded. Both the cis and gauche conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature (−105 to −150 °C) studies of the infrared spectra of the sample dissolved in liquid krypton have been recorded and the enthalpy difference has been determined to be 126±25 cm −1 (1.50±0.30 kJ/mol) but with the cis conformer the more stable form in the rare gas solution. At ambient temperature it is estimated that there is 52±2% of the gauche conformer present. Relatively complete vibrational assignments are proposed for both conformers based on the relative infrared and Raman spectral intensities, infrared band contours, and ab initio predicted frequencies. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and energy differences have been obtained for the cis and gauche conformers from ab initio MP2/631G(d) calculations. Structural parameters and energy differences have also been obtained utilizing the larger 6-311+G(d,p) and 6-311+G(2d,2p) basis sets as well as these bases sets without diffuse functions. The predicted conformer stability and energy differences from these calculations agree with the experimentally determined stability and enthalpy difference except for the calculations at the Hartree–Fock level. Similar calculations by density functional theory (DFT) by the B3LYP method with eight different basis sets up to 6-311+G(2d,2p) gave incorrect conformer stabilities with the gauche conformer more stable by approximately 130 cm −1 from all calculations. Similar DFT calculations for cyclopropylchlorosilane also give incorrect predicted conformational stabilities. The results are discussed and compared to those obtained for some similar molecules.