STUDY OF THE CONFORMERS OF DICHLOROMETHYLMETHYLDIFLUOROSILANE BY VIBRATIONAL SPECTROSCOPY AND AB-INITIO METHODS

Dichloromethylmethyldifluorosilane (CHCl2SiF2CH3) was synthesized for the first time. Raman spectra of the liquid were obtained at various t emperatures between 298 and 163 K, Spectra of the amorphous solid were recorded at 85 K and spectra of crystalline solids cooled by differen t techniques were obtained. The infrared spectra were recorded of the vapor and of the amorphous and crystalline states at liquid nitrogen t emperature. Two different crystals were observed after careful anneali ng 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 compou nd trapped in argon and nitrogen matrices at 5 and 15 K. The compound exists in two different conformations (anti and gauche) due to restric ted rotation of the -CHCl2 group. Approximately 32 of the infrared and /or Raman bands present in the vapor and liquid vanished in one or oth er of the two crystals formed, giving 18 pairs of bands each belonging to one conformer. The remaining vibrational bands were present in bot h crystals, suggesting overlapping bands of the anti and gauche confor mers, Raman temperature studies gave an enthalpy difference Delta H(co nf)degrees = 0 +/- 0.2 kJ mol(-1) between the conformers in the liquid . The bands vanishing in the infrared matrix isolation spectra upon an nealing to ca, 31-34 It suggested that the anti conformer had lower en ergy than gauche in both argon and nitrogen matrices, and the conforma tional barrier was estimated to be 6-7 kJ mol(-1). Ab initio calculati ons at the HF/6-311G level gave optimized geometries, infrared and Ra man intensities and vibrational wavenumbers for the anti and gauche co nformers. An assignment of the vibrational spectra into anti and gauch e conformers was carried out and reasonably good agreement was obtaine d between the experimental and scaled ab initio calculated wavenumbers for the two conformers. (C) 1998 John Wiley & Sons, Led.