A new method to obtain improved structural parameters by supplementing gas-phase electron diffraction (GED) data with restraints based on the results of ab initio calculations is proposed. The procedure involves the use of ab initio parameters with estimated uncertainties as additional observations; this allows previously fixed parameters to refine, with all geometrical parameters included in the final refinement. The refinement of the molecular structure of 2,5-dichloropyrimidine is used as an example to illustrate the principle of this technique. In this simple case, the effects are not very great, but this new approach allowed refinement of all structural parameters. The nine independent structural parameters (r R structure) were found to be: 6)-H( 10)] ) 109.9(12) pm, ∠[N(1)C( 2)N(3)] ) 127.9(4)°, ∠[C(2)N(3)C( 4)] ) 116.3(7)°, and ∠[N(3)C(4)H( 8)] ) 117.2(5)°. All structural parameters were found to be in good agreement with both ab initio and crystallographic values, which are presented for comparison.

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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 [C2/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).

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