Structure analysis restrained by ab initio calculations: The molecular structure of 2,5-dichloropyrimidine in gaseous and crystalline phases

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, die effects are not very great, but this new approach allowed refinement of all structural parameters. The nine independent structural parameters (r(alpha) structure) were found to be: r[C(4)-C(5)] = 139.3(11) pm, r[N(1)-C(6)] = 133.2(4) pm, r[N(1)-C(2)] = 132.5(5) pm, r[C(5)-Cl(9)] = 172.2-(3) pm, r[C(2)-Cl(7)] = 172.8(3) pm, r[C(6)-H(10)] = 109.9(12) pm, angle[N(1)C(2)N(3)] = 127.9(4)degrees, angle[C(2)N(3)C(4)] = 116.3(7)degrees, and angle[N(3)C(4)H(8)] = 117.2(5)degrees. All structural parameters were found to be in good agreement with both ab initio and crystallographic values, which are presented for comparison.