Microwave Spectrum of Methyl Difluorosilane

The microwave spectra of five isotopic species of methyl difluorosilane (CH₃SiHF₂) have been observed. From the rotational constants, the following structural parameters were determined $$\begin{array}{cc}{r}_{\mathrm{FF}}\text{=2.541±0.001\hspace{0.17em}}\mathrm{A}& \\ r_{\mathrm{SiC}}\text{=1.833±0.002\hspace{0.17em}}\mathrm{A}& \angle \mathrm{FSiF}{\text{=106}}^{\circ }{44}^{\prime }{\text{±30}}^{\prime }\\ r_{\mathrm{SiF}}\text{=1.583±0.002\hspace{0.17em}}\mathrm{A}& \mathrm{\angle ;}\mathrm{CSiH}{\text{=116}}^{\circ }{10}^{\prime }{\text{±1}}^{\circ }\\ r_{\mathrm{SiH}}{\mathrm{=r}}_{\mathrm{SiD}}\text{=1.474±0.005\hspace{0.17em}}\mathrm{A}& \angle \mathrm{FSiC}{\text{=109}}^{\circ }{52}^{\prime }{\text{±30}}^{\prime }\\ r_{\mathrm{CH}}\text{=1.09\hspace{0.17em}}\mathrm{A}\mathrm{\hspace{0.17em}(}\mathrm{assumed})& \angle \mathrm{HCH}{\text{=108}}^{\circ }\mathrm{\hspace{0.17em}(}\mathrm{assumed}\mathrm{).}\end{array}$$ A dipole moment of 2.11 debye was obtained from Stark effect measurements. The hindered internal rotation of the methyl group splits some of the rotational lines into doublets with spacings from 0.5 to 2.9 Mc. An analysis of the doublet splitting in three of the isotopic species gives a barrier height of 439 cm^—1 (1255 cal/mole) hindering internal rotation.