The conformers of chloromethylmethyldifluorosilane studied by vibrational spectroscopy andab initio methods

Abstract

Chloromethylmethyldifluorosilane (CH2ClCH3SiF2) was synthesized for the first time. Raman spectra of the liquid were recorded at various temperatures between 298 and 175 K and spectra of the amorphous and crystalline solids were obtained. Infrared spectra were recorded in the vapour, amorphous and crystalline solid phases in the MIR and FIR regions. Additional MIR spectra of the compound isolated in argon and nitrogen matrices were obtained at 10 K. The spectra of chloromethylmethyldifluorosilane showed the existence of two conformers, anti and gauche, present in the vapour and in the liquid. By careful annealing the amorphous solid formed by depositing the vapour on a cold Cu finger (Raman) or on CsI or silicon windows (infrared) at 80 K, two different crystals were formed. One of these, obtained after annealing to 125 K (crystal I), contained the anti conformer; the other, achieved after annealing to ca 170 K (crystal II), contained molecules in the gauche conformation. If the liquid was cooled, crystal II was formed, suggesting this crystal to be stable, whereas crystal I appeared to be metastable. The anti and gauche conformers had approximately the same enthalpy in the liquid, giving a negligible ΔH (gauche–anti) equal to 0.2 ± 0.4 kJ mol−1, but gauche was the conformer of the stable crystal II. Intensity variations in the matrix spectra after annealing indicated that gauche was the low-enthalpy conformer in both matrices and probably also in the vapour. These experimental data make a very reliable assignment of the conformer bands possible. Ab initio calculations were performed at the HF/6–311G* level, from which optimized geometries, infrared and Raman intensities and scaled vibrational wavenumbers for the anti and gauche conformers were obtained. The conformational energy derived was 5.9 kJ mol−1, with gauche being the low-energy conformer. Copyright © 2000 John Wiley & Sons, Ltd.