Conformational Analysis of 1,4-Disilabutane and 1,5-Disilapentane by Combined Application of Gas-Phase Electron Diffraction and ab Initio Calculations and the Crystal Structure of 1,5-Disilapentane at Low Temperatures

The gas-phase structures of the conformers of 1,4-disilabutane and 1,5-disilapentane have been analyzed from electron-diffraction data augmented by flexible restraints derived from ab initio calculations. This allowed the simultaneous refinement of 22 and 29 parameters for 1,4-disilabutane and 1,5-disilapentane, respectively. 1,4-Disilabutane has been found to be present in the vapour predominantly in the anti (A) form (76(2)% from the experiment, 83% predicted by theory). Consistency in the geometries is found between theoretical predictions and experimental findings, except for the torsion angle ∠(SiCCSi) of the gauche (G) conformer [exptl 78.5(21)°, theor 68.0°]. The AA conformer of 1,5-disilapentane was always found to be the lowest energy structure, while some doubt still remains about the ordering of the AG and G(+)G(−) conformers. The AA conformer is found to be the sole form present in the crystal [ C 2/c, a = 15.585(8), b = 4.704(3), c = 9.895(6) Å, β = 95.77(4)°, Z = 4]. Good agreement is found for geometrical parameters determined experimentally in the gas phase and solid state and calculated by ab initio methods. The following values represent the most important distances ( r g /Å) and angles (∠ g /deg) found for the gas phase and crystal structures. 1,4-Disilabutane GED (A/G, esd's correspond to 1σ): r (CSi) 1.882(1)/1.885(1), r (CC) 1.563(5)/1.563(5), r (SiH) both 1.499(3), ∠(CCSi) 110.7(2)/114.4(5), ∠(SiCCSi) 180.0/78.5(21). 1,5-Disilapentane GED [AA/G(+)G(−)]: r (CSi) 1.886(1)/1.888(1), r (CC) 1.537(2)/1.539(2), r (SiH) both 1.487(4), ∠(CCC)114.8(7)/118.8(7), ∠(CCSi) 114.1(4)/116.8(7), ∠(SiCCC) 180.0/60.9(10); X [%, AA/AG/G(+)G(−)] 28(4)/40(5)/26(6). 1,5-Disilapentane XRD: r (CSi) 1.868(2), r (CC) 1.527(2), ∠(CCC) 113.8(2), ∠(CCSi) 115.2(1), ∠(SiCCC) 180.0(1).