SIH STRETCHING FREQUENCIES IN CHLORODISILANES AND AB-INITIO CALCULATIONS OF GEOMETRY, STRETCHING FREQUENCY, VIBRATIONAL INTENSITY AND MULLIKEN AND KING EFFECTIVE ATOMIC CHARGES

Infrared frequencies in the vSiH region observed in mixtures of chloro disilanes are assigned by the joint use of substituent effects, determ ined earlier in Si2H5Cl and 1,1-Si2H4Cl2, and ab initio calculations f or 1,2-Si2H4Cl2, 1,1,2-Si2H3Cl3 and SiH3SiCl3, which together with 1,1 ,2,2-Si2H2Cl4 were identified as being present. vSiH frequencies for a ll the staggered conformers are predicted using harmonic local mode th eory. Ab initio geometries were obtained for the above three compounds at the HF/6-31G level, and for both conformers of 1,2-Si2H4Cl2 at HF /tzvp and MP2/tzvp levels. Alpha or gauche chlorine substitution short ens both SiH and SiCl bonds, the former reflecting vSiH changes. Trans chlorine substitution shortens SiCl bonds slightly, but has no effect on SiH bonds. The local mode behaviour of vSiH vibrations extends to both infrared and Raman intensities. Ab initio-based atomic polar tens ors are used to obtain dipole derivatives, dv/dr, for the SiH and SiCl bonds and King effective atomic charges, xi, for all atoms. The chang es in the latter are compared with changes in the Mulliken atomic char ges and a close connection is found in the case of gauche chlorine sub stitution.