THE CONFORMERS OF CHLOROMETHYL DIMETHYL FLUOROSILANE STUDIED BY VIBRATIONAL SPECTROSCOPY AND AB-INITIO METHODS

Chloromethyl dimethyl fluorosilane (CH2Cl-(CH3)(2)SiF) 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 c rystalline solids were recorded. The infrared spectra of the vapour, o f the amorphous and crystalline states were obtained between 4000 and 50 cm(-1). The compound was mixed with argon and nitrogen and the vapo ur mixture was deposited on a CsI window at 5 and at 15 K, the infrare d spectra were recorded in the range 4000-400 cm(-1) before and after annealing. The spectra reveal that the compound exists as anti and,gau che conformers in the vapour, liquid, in the unannealed matrices and i n the amorphous solid. Six cases were observed when infrared and Raman bands present in the fluid phases vanished after crystallization. Ram an temperature studies in the liquid gave Delta(conf)H=0.2+/-0.15 kJ m ol(-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 n itrogen 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 i ntensities and vibrational frequencies for the anti and gauche conform ers. After scaling, a reasonably good agreement between the experiment al and calculated wavenumbers for the anti and gauche conformers was o btained. (C) 1998 Elsevier Science B.V.