POSIT – a method for structure determination of small partially known molecules from powder diffraction data.Structure of 6-methyl-1,2,3,4-tetrahydropyrimidine-2,4-dione (6-methyluracil)

POSIT is a computer program package for the determination of the approximate structure of small organic molecules using known molecular fragments. The orientation and the translation vector of this fragment are determined and refined by a special R-value analysis. In contrast to other molecular replacement methods, only a few low-order strong reflections are needed to start the structure analysis. Therefore, this method is suitable for the determination of crystal structures of small rigid or semi-rigid organic molecules without heavy atoms from powder diffraction data. C₅H₆N₂O₂, M_r = 126.11, monoclinic, P2₁/c, a = 4.5137 (7), b = 10.9888 (12), c = 11.7224 (9) Å, β = 97.554 (8)°, V = 576.39 (7) ų, Z = 4, D_x = 1.453 Mg m^-3, λ (Cu Kα) = 1.54178 Å, μ = 0.987 mm^-1, F(000) = 264, T = 297 K, final R = 0.04 for 165 approximate structure factors obtained from a powder diffraction measurement. The molecules were found in the diketo form. They are linked by a two-dimensional network of hydrogen bonds forming layers parallel to the yz plane.