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 augme nted by flexible restraints derived from ab initio calculations. This allowed the simultaneous refinement of 22 and 29 parameters for 1,4-di silabutane and 1,5-disilapentane, respectively. 1,4-Disilabutane has b een found to be present in the vapour predominantly in the anti (A) fo rm (76(2)% from the experiment, 83% predicted by theory). Consistency in the geometries is found between theoretical predictions and experim ental findings, except for the torsion angle angle(SiCCSi) of the gauc he (G) conformer [exptl 78.5(21)degrees, theor 68.0 degrees]. The AA c onformer of 1,5-disilapentane was always found to be the lowest energy structure, while some doubt still remains about the ordering of the A G and G(+)G(-) conformers. The AA conformer is found to be the sole fo rm present in the crystal [C2/c, a = 15.585(8), b = 4.704(3), c = 9.89 5(6) Angstrom, beta = 95.77(4)degrees, Z = 4]. Good agreement is found for geometrical parameters determined experimentally in the gas phase and solid state and calculated by nb initio methods. The following va lues represent the most important distances (r(g)/Angstrom) and angles (angle(g)/deg) found for the gas phase and crystal structures. 1,4-Di silabutane GED (A/G, esd's correspond to 1 sigma): r(CSi) 1.882(1)/1.8 85(1), r(CC) 1.563(5)/1.563(5), r(SiH) both 1.499(3), angle(CCSi) 110. 7(2)/114.4(5), angle(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), angle(CCC)114.8(7)/118.8(7), angle(CCSi)114.1(4)/116.8( 7), angle(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), angle(CCC) 113.8(2), angle(CCSi) 115.2(1), angle(siCCC) 180.0(1).