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

Quantum-chemical studies of methyl and silyl torsional frequencies and the structures of disilylsulphide and trisilylphosphine

Torsional frequencies of methyl and silyl groups occurring in a range of molecules have been calculated by HF, B3LYP and MP2 methods using several basis sets. Linear correlations between calculated and observed values are derived and used to predict unobserved or dubious frequencies. The current experimental value for the E torsion in trimethylphosphine is questioned. The relative merits of the B3LYP and MP2 methods are explored. MP2 calculations can show wide variation with respect to basis set. In cases where two or more silyl groups are attached to a common atom, as in disilylsulphide (SiH3)2S, disilylmethane (SiH3)2CH2, trisilylmethane (SiH3)3CH and tetrasilylmethane (SiH3)4C, marked differences occur between B3LYP and MP2 estimates. These may be linked to concomitant differences in conformation or potential barrier restraining internal rotation. In disilylmethane the B3LYP results agree much better with experiment than those from the MP2 method. HF and B3LYP calculations for disilylsulphide and trisilylphosphine give normal C2v and C3v equilibrium structures, respectively, but in MP2 structures the silyl groups are twisted through 6-13 degrees yielding C2 and C3 configurations. It may be possible to distinguish between these structures through the observation of isolated SiH stretching frequencies in the spectra of fully deuterated materials. Dispersion forces could contribute to the twisting calculated by the MP2 method. Further studies of the microwave and vibrational spectra of disilylsulphide and trisilylphosphine isotopomers are needed.

A quantum-chemical study of the structure, vibrations and SiH bond properties of disilylamine, NH(SiH3)2

Quantum-chemical calculations at HF, MP2 and B3LYP levels with 6-31G* and 6-311G** basis sets are reported for disilylamine, NH(SiH3)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 HNSi2 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(CH3)(SiH3)2. Unobserved frequencies due to the v(s)NSi2, deltaNSi2 and delta(perpendicular)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 vSiH band in gas-phase samples of both NH(SiH3)2 and NH(SiH3)(SiD3) is suggestive of signal averaging due to internal rotation. The frequencies v(is)SiH, 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)-sigma*(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. vSiH-vSiH 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 delta mu/delta 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.