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

Abstract

New infrared spectra are reported for variously labelled trisilylamines. Quantum-chemical (QC) calculations of structure and force field have been made at HF, MP2 and B3LYP levels, each with the 6-31G* and 6-311G** basis sets. At each level, a minimum in the potential surface occurs at the C 3h configuration. No evidence was found for a significant variation in SiH bond length with orientation. The appearance of two bands in the infrared spectrum of N(SiH 3) 3 in the 2 νSiH region is explained by local mode theory in terms of transitions to (200) and (110) levels. In the gas phase, signal averaging appears to occur in the νSiH region in the infrared spectrum, but not in the Raman. In solid films, both IR and Raman spectra indicate the presence of a range of SiH bond strengths, corresponding to an absence of any site symmetry. Each complete QC calculated force field was fitted to the frequencies observed for N(SiH 3) 3 and N(SiD 3) 3, using nine independent scale factors. An interaction force constant between ν asNSi 3 and δ sSiH 3 motions required further adjustment. Unobserved frequencies in the d 0 and d 9 species are predicted. The out-of-plane skeletal bending mode is expected to lie between 170 and 200 cm −1. Unscaled SiH 3 torsional frequencies vary from 64 cm −1 upwards. The effect of the presence of three internal rotors on the spectra throughout calls for theoretical study.