Infrared and Raman spectra of monochloro- and monobromodisilanes, a scaled ab initio force field, intensities, atomic polar tensors and effective charges for Si2H5Cl

Infrared and Raman spectra are reported for Si2H5Cl, Si2D5Cl, Si2H5Br and Si2D5Br. These include approximate infrared intensities for Si2H5Cl. All the fundamentals of the chloro compounds, except the torsion, are securely assigned, and nearly all those of the bromo ones. The force field and spectral intensities of Si2H5Cl were calculated ab initio using a 6-31G* basis set, and the force field scaled to fit the d0, d5 and nu(is)SiH data, using 14 independent scale factors. Significant changes in normal coordinate occur on scaling. The SiH stretching part of this force field agrees well with an earlier harmonic local mode force field, although differences occur in the normal coordinates. The 6-31G* atomic polar tensors for the H and Cl atoms indicate that the dipole derivatives for the SiH and SiCl bonds lie close to the bond directions. Both H and Cl atoms behave as if negatively charged in all motions. Comparisons of nuSiH intensities and frequencies, effective charges and Mulliken atomic charges are made across the series SiH4-Si2H6-Si2H5Cl. These identify significant effects by chlorine on the gauche SiH bond, but virtually none on the trans one.