Vibrational spectroscopic studies, conformations and ab initio calculations of n-propyltrichlorosilane

Infrared spectra of n-propyltrichlorosilane (CH3CH2CH2SiCl3) were obtained in the vapour, amorphous and crystalline solid phases in the range 4000-50 cm(-1). Additional spectra in argon and nitrogen matrices at 5 K were recorded before and after annealing to 20-36 K. Raman spectra of the compound as a liquid were recorded at various temperatures between 300 and 153 K and spectra of the amorphous and crystalline solids were obtained. The spectra revealed the existence of two conformers (anti and gauche) in the fluid phases and in the matrices. When the vapour was shock-frozen on a cold-finger at 78 K and subsequently annealed to 120-130 K, 10 Raman bands and eight IR bands of the liquid were reduced in intensity in the amorphous phase and vanished in the crystal. Frequent intensity variations were observed in the infrared spectra of argon and nitrogen matrices before and after annealing. These spectra revealed the existence of one conformer (anti) in the crystal. From intensity variations of four independent pairs of anti and gauche bands in the Raman spectra between 300 and 150 K, an average value DeltaHdegrees (gauche-anti) = 4.6 +/- 0.3 kJ mol(-1) was obtained in the liquid. Annealing experiments on n-propyltrichlorosilane in the matrices demonstrate that the gauche bands vanish after annealing, revealing that the anti conformer has the lower energy. The spectra of both conformers were interpreted in detail. Ab initio and DFT calculations gave optimized geometries, infrared and Raman intensities and scaled vibrational wavenumbers for the anti and gauche conformers. The conformational energy difference derived was 7.6 and 6.4 kJ mol(-1) for the HF/6-311G* and B3LYP/6-311G* basis sets, respectively, with anti being the low-energy conformer. Copyright (C) 2004 John Wiley Sons, Ltd.