1,1,1,3,3,3-Hexabromotrisilane: structure and conformation determined by gas-phase electron diffraction, ab initio molecular orbital and molecular mechanics calculations, and vibrational spectroscopy

Abstract

The molecular structure of 1,1,1,3,3,3-hexabromotrisilane at 140°C has been studied using gas-phase electron diffraction. The two SiBr 3 groups are both staggered relative to the central SiH 2 group, but a twist of about 13° of the SiBr 3 groups relative to the exactly staggered position reduces the symmetry to C 2. From the vibrational spectra a distinction between C 2 or C 2v symmetry is possible, and point group C 2v can be ruled out by arguments based on the selection rules. Bond lengths ( r g) and valence angles (∠ a) are: r(Si Si) = 2.344(18) Å, r(Si Br) = 2.205(4) Å ∠SiSiSi = 112.9(19)°, ∠〈BrSiBr〉 = 109.6(6)° (average BrSiBr angle), and ∠〈SiSiBr angle). Error limits are given as 2 σ where σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data. The infrared and Raman vibrational spectra have been assigned using normal coordinate calculations as well as ab initio calculations, and harmonic force constants are reported. In addition molecular mechanics and ab initio (RHF/SBK and RHF/STO-3G) calculations were performed to assist the structural analysis and to compare with the diffraction results.