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

The gas-phase structures of the disilanes 1,1,2,2-tetrakis(trimethylsilyl)disilane [(Me3Si)(2)HSiSiH(SiMe3)(2)] (1) and 1,1,2,2-tetrakis(trimethylsilyl)dimethyldisilane [(Me3Si)(2)MeSiSiMe(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 C-2-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)(2)RSiSiR(SiMe3)(2) [R = H (1), CH3 (2)] the lowest energy conformers were found by GED to have RSiSiR dihedral angles of 87.7(17)degrees for 1 and -47.0(6)degrees 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.