Microwave Spectrum, Structure, and Internal Barrier of Methyl Silane

The microwave spectrum of methyl silane has been reinvestigated. Seven symmetric top and ten asymmetric top isotopic species have been studied. These molecules, together with the six symmetric tops studied by Lide and Coles, yield a total of thirty‐three rotational constants. A least‐squares analysis of these constants gave the following structural parameters: $$\begin{array}{cc}{\mathrm{\hspace{0.17em}\hspace{0.17em}\; \hspace{0.17em}r}}_{\mathrm{SiC}}\text{=1.8668±0.0005\hspace{0.17em}A,}& {\mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}r}}_{\mathrm{CH}}\text{=1.093±0.005\hspace{0.17em}A,}\\ {\mathrm{\hspace{0.17em}\hspace{0.17em}\; \hspace{0.17em}r}}_{\mathrm{SiH}}\text{=1.485±0.005\hspace{0.17em}}\mathrm{A},& \angle \mathrm{HCH}{\text{=107}}^{\circ }{40}^{\prime }{\text{±30}}^{\prime },\\ \angle \mathrm{HSiH}{\text{=108}}^{\circ }{15}^{\prime }{\text{±30}}^{\prime }.& \end{array}$$ Analysis of the spectra of the isotopic species CH₂D–SiH₂D and CH₂D–SiHD₂ shows conclusively that methyl silane in its equilibrium configuration has the methyl group staggered with respect to the silyl group. Certain transitions in the asymmetric top isotopic species, e.g., CH₃SiH₂D, are split into doublets because of coupling of over‐all and internal rotation. These splittings were used to determine the barrier to internal rotation. The form of the potential hindering internal rotation was taken to be V = ½V₃(1 — cos3α)+½V₆(1 — cos6α), giving the result V₃ = 1700±100 cal, V₆∼0–150 cal. This is in reasonable agreement with the result of Kivelson's analysis of the J = 0→1 torsional satellites of CH₃–SiH₃.