Infrared and quantum-chemical studies of the structure and vibrations of methyldisilylamine

Infrared spectra are reported from labelled species of methyldisilylamine (N(CH3)(SiH3)(2)) in the gas and solid phase. Quantum-chemical (QC) calculations of structure and force field have been carried out at HF, MP2 and B3LYP levels using 6-31G* and 6-311G** basis sets. The equilibrium structure belongs to the point group C, with the plane of symmetry at right angles to the NSi2 angle and a slight non-planarity of the CNSi2 moiety (C-s, (perpendicular to)). Spectra in the vSiH region however suggest an effective C-2v structure, with two distinct types of SiH bond. Two strong SiH bonds lie in the skeletal plane, eclipsing each other. The single v(is)CH band in the CHD2 species suggests the presence of signal averaging due to internal rotation of the methyl group. The QC calculations indicate the presence of a lower barrier to internal rotation for the methyl group than for the silyl one. Scaled QC-based force fields lead to a number of reassignments of previous Raman and infrared spectra. However, difficulty remains in interpreting the region below 200 cm(-1) where out-of-plane skeletal bending and torsional modes are expected. In the scaling procedure, a maximum number (19) of factors for different types of motion was determined. Significant variations are found in these factors for similar types of coordinate. Symmetric SiN stretching requires a factor similar to7% greater than that for asymmetric SiN stretching. The spread of these 19 factors is similar in each force field calculation. This indicates that DFT-based force fields require as much careful scaling as do HF or MP2 ones. However, the average DFT factor lies closest to unity. (C) 2002 Elsevier Science B.V. All rights reserved.