VIBRATIONAL-SPECTRA IN THE NU(SIH) AND NU(SID) REGIONS OF CHLORO AND BROMODISILANES AND AB-INITIO GEOMETRY STUDIES OF C2H5CL, SI2H5CL AND 1,1-SI2H4CL2

Infrared measurements in the gas phase are reported for the v(SiH) and v(SiD) regions of Si2H5X, Si2D5X, 1,1-Si2H4X2 and 1,1-Si2D4X2 Species where X=Cl, Br. Incomplete Raman data have also been obtained. All three possible isolated SiH stretching frequencies are observed in the spectra of the Si2D4X2 samples, but only two from the Si2D5X ones. The missing nu(is)(SiH) values are obtained by use of the frequency sum rule, and by harmonic local mode force field treatments of all the available nu(SiH) and nu(SiD) data, using a procedure previously tested on disilane. Ab initio calculations of the geometries of C2H5Cl, Si2H5Cl and 1,1-Si2H4Cl2 using the 6-31G* basis set are reported. Trends in r(e)(CH) or r(e)(SiH) values reflect trends in nu(is)(CH) or nu(is)(SiH) ones. The alpha, trans and gauche effects of halogen are similar in CH and SiH compounds, although smaller in the latter. In both cases, ab initio calculations predict larger effects than are observed in the spectra, especially for the alpha effect of halogen. A kinetic isotope effect in the halogenation of disilane may occur. Reassignment of earlier spectra of disilyl iodide species is proposed.