A quantum-chemical study of the structure, vibrations and SiH bond properties of disilylamine, NH(SiH 3) 2

Abstract

Quantum-chemical calculations at HF, MP2 and B3LYP levels with 6–31G* and 6–311G** basis sets are reported for disilylamine, NH(SiH 3) 2. The equilibrium structure is found to vary with both level and basis set, all but one of the structures exhibiting a small lack of planarity of the HNSi 2 system. The barrier to inversion, however, is found to be very low, at most 38 cm −1. Vibration frequencies and intensities are calculated. The frequencies are scaled, where possible, either using updated infrared data or with the aid of factors transferred from N(CH 3)(SiH 3) 2. Unobserved frequencies due to the ν sNSi 2, δNSi 2 and δ ⊥NH modes are predicted near 610, 210 and 360 cm −1, respectively. The lower silyl torsion lies below 40 cm −1. The appearance of a single broad νSiH band in gas-phase samples of both NH(SiH 3) 2 and NH(SiH 3)(SiD 3) is suggestive of signal averaging due to internal rotation. The frequencies ν isSiH, infrared intensities and Raman scattering activities of the bands due to an isolated SiH bond in an otherwise deuterated species are calculated and correlated with the torsional angle of this bond and with the Mulliken charge on the hydrogen atom. The strength of the bond is a minimum, and the infrared intensity and Raman scattering activity are maxima, when the bond direction is roughly orthogonal to the skeletal plane. A major part of the frequency and intensity variations is attributed to n p(N)–σ*(SiH)) hyperconjugation which, NBO calculations show, reaches a maximum for this conformation. However, systematic smaller variations are found for SiH bonds lying in the skeletal plane, which reflect the proximity of the other silyl group and only partly correlate with Mulliken charge. νSiH–νSiH interaction force constants, f′, are calculated for pairs of SiH bonds in different silyl groups and compared with the corresponding dipole–dipole potential energy, the latter calculated using a classical treatment of the interaction between point dipoles arising from ∂μ/∂ r for the SiH bonds involved. The gradient of the correlation is very close to that expected from the theory, but a negative intercept indicates the presence of additional factors.