Gas-phase structures of sterically crowded disilanes studied by electron diffraction and quantum chemical methods: 1,1,2,2-tetrakis(trimethylsilyl)disilane and 1,1,2,2-tetrakis(trimethylsilyl)dimethyldisilane

Abstract

The gas-phase structures of the disilanes 1,1,2,2-tetrakis(trimethylsilyl)disilane [(Me₃Si)₂HSiSiH(SiMe₃)₂] (1) and 1,1,2,2-tetrakis(trimethylsilyl)dimethyldisilane [(Me₃Si)₂MeSiSiMe(SiMe₃)₂] (2) have been determined by density functional theoretical calculations and by gas electron diffraction (GED) employing the SARACEN method. For each of 1 and 2 DFT calculations revealed four C₂-symmetric conformers occupying minima on the respective potential-energy surfaces; three conformers were estimated to be present in sufficient quantities to be taken into account when fitting the GED data. For (Me₃Si)₂RSiSiR(SiMe₃)₂ [R = H (1), CH₃ (2)] the lowest energy conformers were found by GED to have RSiSiR dihedral angles of 87.7(17)° for 1 and −47.0(6)° for 2. For each of 1 and 2 the presence of bulky and flexible trimethylsilyl groups dictates many aspects of the geometric structures in the gas phase, with the molecules often adopting structures that reduce steric strain.