MOLECULAR-STRUCTURES AND CONFORMATIONAL COMPOSITION OF TRISILANE AND TETRASILANE BY GAS-PHASE ELECTRON-DIFFRACTION

The gas-phase electron diffraction patterns of trisilane and normal tetrasilane have been recorded with an all-glass inlet system and a nozzle temperature of 23 +/- 2 degrees C. The molecular structure of trisilane was optimized and the valence force field calculated by nb initio MO calculations at the 6-31G**/MP2 level, and the structures and valence force fields of anti and gauche conformers of tetrasilane were calculated at the 6-31G*/SCF level. The force fields were scaled and used to calculate root mean-square vibrational amplitudes and correction terms for molecular vibrations. Refinement of a geometrically consistent r(alpha)-structure of Si3H8 yielded a Si-Si bond distance of r(alpha) = 233.2(2) pm and a valence angle of L(alpha)SiSiSi = 110.2(4)degrees. Refinements of a mixture of geometrically consistent r(alpha)-models of gauche and anti conformers of Si4H10 yielded the bond distances (r(alpha)) Si(1)-Si(2) = 233.5(3) and Si(2)-Si(3) = 234.0(3) pm and the valence angle L(alpha)SiSiSi = 109.6 degrees. The mole fraction of the gauche conformer was chi = 68(9)% corresponding to a free energy difference of 0.2(1.1) kJ mol(-1) in favor of gauche. Introduction of the thermal vibration correction terms of tetrasilane calculated from the scaled quantum-mechanical force field led to significantly poorer agreement between experimental and calculated intensities.