Microwave spectrum and conformation of n-propyltrifluorosilane

The conformational properties of gaseous n-propyltrifluorosilane (CH3CH2CH2SiF3) have been investigated by microwave spectroscopy and high-level quantum chemical calculations. The microwave spectrum was investigated in the 20-62 GHz spectral range at a temperature of −78 °C. The spectra of the ground vibrational state and three vibrationally excited states of one conformer having an antiperiplanar conformation of the C-C-C-Si chain of atoms were assigned. No evidence for the existence of the synclinal (gauche) conformer was seen in the microwave spectrum. It is concluded that the synclinal form is at least 3.5 kJ/mol less stable than the antiperiplanar conformer in the gas phase. Density functional theory calculations have been performed for the system mainly to predict the effects of centrifugal distortion. The G3 quantum chemical method has been used to test the ability of this method to predict the energy difference between the synclinal and antiperiplanar conformers.