1,1,2-Triiododisilane (I2HSi–SiH2I): molecular structure, internal rotation and vibrational properties determined by gas-phase electron diffraction, infrared and Raman spectroscopy, and ab initio molecular orbital- and density functional calculations

Abstract

The molecular structure, internal rotation and vibrational properties of 1,1,2-triiododisilane (TIDS), I2HSi–SiH2I, have been studied using gas phase electron diffraction (GED) data at an average temperature of 52°C, together with infrared and Raman spectroscopy, and ab initio molecular orbital- and density functional calculations. The title compound exists in the gas and liquid phases as a mixture of two minimum conformers, anti, with a torsional angle φ(HSiSiI)=180°, and gauche, with a torsional angle φ(HSiSiI)≈80°. Discrepancies were found between the experimental results and the theoretical calculations regarding conformational stability. The GED analysis, using a cosine potential function in describing the torsional motion, gives about 51% contribution of the anti conformer, while the corresponding spectroscopic result contributes about 64%. The theoretical calculations give the gauche conformer a lower energy of average value 1.1kcalmol−1, corresponding to 90% of this conformer in the gaseous mixture. Some structural parameter values obtained from the GED refinements, using results from the theoretical calculations as constraints, are as follows (torsional vibrational average values with estimated 2σ uncertainties): Bond lengths (rg): r(Si–Si)=2.329(12) Å, r (Si–I) =2.449(3) Å (average value of the three Si–I bonds), r(Si–H)=1.527Å (estimated value). Bond angles (∠α): ∠(SiSiI)=109.9(4)° (average value), ∠ISiI=110.5(4)°, ∠ISiH=109.0(2)°. The torsional cosine potential-function parameters V1, V2 and V3 were obtained from the GED data analysis. Vibrational spectra were presented and analysed, aided by normal coordinate calculations and ab initio results.