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

Abstract

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: C=C1.347 ASi–C=C122∘53′ Si–H1.475H–Si–H108∘42′ C–H(cis)1.097C=C–H(cis)120∘18′C–H(trans)1.097 (assumed)C=C–H(trans)120∘38′C–H1.094C=C–H117∘59′Si–C1.853By 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 μa = 0.648D, μb = 0.133D, and μ = 0.66D. The resultant dipole moment makes an angle of 11±2° with the a axis of SiH3CHCH2.