Study of the conformers of dichloromethylmethyldifluorosilane by vibrational spectroscopy and ab initio methods

Abstract

Dichloromethylmethyldifluorosilane (CHCl2SiF2CH3) was synthesized for the first time. Raman spectra of the liquid were obtained at various temperatures between 298 and 163 K. Spectra of the amorphous solid were recorded at 85 K and spectra of crystalline solids cooled by different techniques were obtained. The infrared spectra were recorded of the vapor and of the amorphous and crystalline states at liquid nitrogen temperature. Two different crystals were observed after careful annealing of the amorphous solid to 202 K (crystal I) and 208 K (crystal II). Both the Raman and infrared spectra of these crystals were different, interpreted as different conformers being present in the two crystal lattices. When the liquid was cooled, crystal II was invariably formed, suggesting this crystal to be stable, whereas crystal I appeared to be metastable. Additional infrared spectra were obtained of the compound trapped in argon and nitrogen matrices at 5 and 15 K. The compound exists in two different conformations (anti and gauche) due to restricted rotation of the —CHCl2 group. Approximately 32 of the infrared and/or Raman bands present in the vapor and liquid vanished in one or other of the two crystals formed, giving 18 pairs of bands each belonging to one conformer. The remaining vibrational bands were present in both crystals, suggesting overlapping bands of the anti and gauche conformers. Raman temperature studies gave an enthalpy difference ΔHconf°=0±0.2 kJ mol-1 between the conformers in the liquid. The bands vanishing in the infrared matrix isolation spectra upon annealing to ca. 31–34 K suggested that the anti conformer had lower energy than gauche in both argon and nitrogen matrices, and the conformational barrier was estimated to be 6–7 kJ mol-1. Ab initio calculations at the HF/6–311G* level gave optimized geometries, infrared and Raman intensities and vibrational wavenumbers for the anti and gauche conformers. An assignment of the vibrational spectra into anti and gauche conformers was carried out and reasonably good agreement was obtained between the experimental and scaled ab initio calculated wavenumbers for the two conformers. © 1998 John Wiley & Sons, Ltd.