Microwave Spectrum, Structure, Dipole Moment, and Internal Barrier of Vinyl Silane

The structure of vinyl silane has been determined by studying the microwave spectra of each of its singly substituted isotopic species. Using Kraitchman's equations the following structural parameters were calculated: $$\begin{array}{cccc}\mathrm{C}=\mathrm{C}& \text{1.347\hspace{0.17em}}\mathrm{A}& \mathrm{Si}–\mathrm{C}=\mathrm{C}& {122}^{\circ }{53}^{\prime }\mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}\\ \mathrm{Si}–\mathrm{H}& \text{1.475}& \mathrm{H}–\mathrm{Si}–\mathrm{H}& {108}^{\circ }{42}^{\prime }\mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}\\ \mathrm{C}–\mathrm{H}\mathrm{(cis)}& \text{1.097}& \mathrm{C}=\mathrm{C}–\mathrm{H}\mathrm{(cis)}& {120}^{\circ }{18}^{\prime }\\ \mathrm{C}–\mathrm{H}\mathrm{(trans)}& \text{1.097\hspace{0.17em}(}\mathrm{assumed})& \mathrm{C}=\mathrm{C}–\mathrm{H}\mathrm{(trans)}& {120}^{\circ }{38}^{\prime }\\ \mathrm{C}–\mathrm{H}& \text{1.094}& \mathrm{C}=\mathrm{C}–\mathrm{H}& {117}^{\circ }{59}^{\prime }\\ \mathrm{Si}–\mathrm{C}& \text{1.853}& & \end{array}$$ By studying the isotopic species containing an asymmetric silyl group the equilibrium conformation was found to be staggered, i.e., the silyl hydrogens are staggered with respect to the hydrogen on the central carbon. The symmetry axis of the silyl group was found to be tilted by an angle of 1°49′ from the Si–C bond axis towards the double bond. Comparison of the Si–C bond distance of vinyl silane with that of methyl silane indicates that recent estimates of the effect of changes in hybridization on the covalent radius of carbon are much too large. The barrier to internal rotation for vinyl silane has been calculated from splittings of groundstate rotational transitions of SiH₃CHCH₂ and splittings of first‐excited state transitions of SiH₃CHCH₂ and SiD₃CHCH₂, and also from Stark effect measurements on the K = 1, J = 2←1 transitions of SiH₃CHCH₂. The various calculations are all in good agreement, and the best value for the barrier is found to be 1500±30 cal/mole. Quadratic Stark effect measurements on the J = 2←1 transitions gave μ_a = 0.648D, μ_b = 0.133D, and μ = 0.66D. The resultant dipole moment makes an angle of 11±2° with the a axis of SiH₃CHCH₂.