Ab Initio Studies of Disilazanes: Structures and Vibrational Properties of Hexachloro-, Hexamethyl-, and Tetrachlorodisilazane

Abstract

The structures of hexachlorodisilazane, NH(SiCl3)2, and hexamethyldisilazane, NH(SiMe3)2, have been calculated at HF/6-31G* and MP2/6-31G* levels. Both contain planar HNSi2 skeletons and show the expected staggering of the substituents as seen along the Si‚‚‚Si direction, as previously found in the electron diffraction structure of the hexamethyl compound. Unlike the latter, however, the ab initio structures both belong to the C2 point group. Force fields for these two molecules and for tetrachlorodisilazane, NH(SiHCl 2)2, were calculated at the HF level and scaled to produce vibration frequencies for comparison with previously obtained spectra. For NH(SiCl3)2, all but five of the fundamentals are satisfactorily assigned. Seven scale factors could be refined, other factors being transferred from model compounds. For the hexamethyl compound, the interpretation was less detailed. In all three molecules the out-of-plane NH bending motion is found to be associated with fundamentals around or below 400 cm -1 , far lower than previous assignments. The force constant for this motion is greatly reduced as chlorine substituents are replaced by methyl groups, which shows the sensitivity of the stability of the planar system to the inductive effect of substituents. For tetrachlorodisilazane, NH(SiHCl2)2, earlier spectra were reanalyzed and evidence was found for the presence of two conformers, as suggested by previous ab initio investigations. The vibrational properties of the SiH bonds were explored by extending the ab initio calculations to a series of nonequilibrium structures, with the object of mapping variations in the SiH bond with change in torsional angle. This bond is strongest in the skeletal plane and weakest at right angles to the latter, which is in accordance with an n(N)-U*(Si-H) orbital interaction. However the orientation of the other silyl group also influences SiH bond strength. Long-range interactions are found between the stretching motions of both SiH and SiCl bonds. The former appear to arise from dipole-dipole interactions between the SiH bonds, but the source of the latter remains obscure. The form of the observed OSiH spectrum suggests that signal averaging conceals the splittings expected from both interactions and bond strength variation. Electrical properties of the SiH and SiCl bonds are examined.