The conformers of chloromethyl dimethyl fluorosilane studied by vibrational spectroscopy and ab initio methods

Abstract

Chloromethyl dimethyl fluorosilane (CH 2Cl(CH 3) 2SiF) was synthesized and investigated by vibrational spectroscopy and by ab initio quantum chemical methods. Raman spectra of the liquid were obtained at seven temperatures between 295 and 174 K, and spectra of the amorphous and crystalline solids were recorded. The infrared spectra of the vapour, of the amorphous and crystalline states were obtained between 4000 and 50 cm −1. The compound was mixed with argon and nitrogen and the vapour mixture was deposited on a CsI window at 5 and at 15 K, the infrared spectra were recorded in the range 4000-400 cm −1 before and after annealing. The spectra reveal that the compound exists as anti and gauche conformers in the vapour, liquid, in the unannealed matrices and in the amorphous solid. Six cases were observed when infrared and Raman bands present in the fluid phases vanished after crystallization. Raman temperature studies in the liquid gave Δ conf H = 0.2 ± 0.15 kJ mol −1. The gauche conformer was the low energy conformer and the only one present in the crystal. The IR bands vanishing in the argon and nitrogen matrix spectra after annealing to ca. 28–34 K, suggested that the anti conformer had a lower energy than the gauche in both matrices. The conformational barrier was estimated to be ca. 7 kJ mol −1. Ab initio calculations at the HF/3–21G∗, HF/6–31G∗, HF/6–311G∗ and MP2/6–31G∗ levels of approximation gave optimized geometries, IR and Raman intensities and vibrational frequencies for the anti and gauche conformers. After scaling, a reasonably good agreement between the experimental and calculated wavenumbers for the anti and gauche conformers was obtained.