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Infrared spectra are reported from labelled species of methyldisilylamine (N(CH 3 )(SiH 3 ) 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 s with the plane of symmetry at right angles to the NSi 2 angle and a slight non-planarity of the CNSi 2 moiety ( C s ,⊥ ). Spectra in the ν SiH region however suggest an effective C 2 v structure, with two distinct types of SiH bond. Two strong SiH bonds lie in the skeletal plane, eclipsing each other. The single ν is CH band in the CHD 2 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 ∼7% 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. |
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