Microwave spectrum, rs structure,dipole moment,and internal rotation of methyl fluorosilane

Microwave spectra of methyl fluorosilane and its 20 isotopic species were measured. In order to determine the most reliable r_s structure, atom coordinate values were obtained by solving the Kraitchman equations from several sets of the substituted and parent species and by averaging the solutions. For unreliable Kraitchman coordinate values, several trials were made in order to fix the values. The second difference method was also applied to the unreliable Kraitchman values. The dipole moment and its direction in the molecule were obtained by Stark-effect measurements for the normal and two deuterated species. From the A-E splittings of the observed spectra in the ground state for nine species, the barrier to internal rotation and the direction of the top axis of the methyl group were determined. It is noted that the top axes obtained from the structure and from the analysis of the A-E splittings do not coincide with each other. From the structural analysis the methyl group is found to tilt toward two hydrogen atoms on the silicon atom by about 1°45′, whereas the analysis of the A-E splittings shows the methyl group tilting toward the fluorine atom by about 37′. Comparison of the results was made among methyl fluorosilane, ethyl fluorosilane, ethyl fluoride, and methyl fluorogermane.