Abstract
With only a 2.6 Å resolution laboratory powder diffraction pattern of the θ phase of Pigment Yellow 181 (P.Y. 181) available, crystal-structure solution and Rietveld refinement proved challenging; especially when the crystal structure was shown to be a triclinic dimethylsulfoxide N-methyl-2-pyrrolidone (1:1:1) solvate. The crystal structure, which in principle has 28 possible degrees of freedom, was determined in three stages by a combination of simulated annealing, partial Rietveld refinement with dummy atoms replacing the solvent molecules and further simulated annealing. The θ phase not being of commercial interest, additional experiments were not economically feasible and additional dispersion-corrected density functional theory (DFT-D) calculations were employed to confirm the correctness of the crystal structure. After the correctness of the structure had been ascertained, the bond lengths and valence angles from the DFT-D minimized crystal structure were fed back into the Rietveld refinement as geometrical restraints (`polymorph-dependent restraints') to further improve the details of the crystal structure; the positions of the H atoms were also taken from the DFT-D calculations. The final crystal structure is a layered structure with an elaborate network of hydrogen bonds.
Highlights
With hardware and software becoming increasingly more powerful, it is possible to solve crystal structures even from relatively limited experimental diffraction data
Quantum-mechanical calculations, on the other hand, are able to reproduce molecular geometries with high accuracy nowadays, and by energyminimizing the complete crystal structure, as opposed to a single molecule in vacuum, even the subtle effects of crystal packing on molecular geometry are taken into account
In the case of -P.Y. 181, the DFT-D calculations were primarily needed because of the scarcity of the experimental data; in general, checking crystal structures from powder diffraction data against information available from independent sources is always recommended, even in cases where experimental data of acceptable resolution are available
Summary
With hardware and software becoming increasingly more powerful, it is possible to solve crystal structures even from relatively limited experimental diffraction data This opens up new possibilities for crystal structure determination in the absence of high-quality data, e.g. when dealing with highly unstable phases or when the sample shows poor crystallinity (David et al, 2005; Schmidt et al, 2005). Due to peak overlap and due to reduced intensities because of the Lorentz-polarization factor in the high 2 region of a powder pattern, the most useful information in a powder diffraction pattern is contained in the doi:10.1107/S2052520615000724 89 research papers low 2 region (below about 2.5 Areal-space resolution, for copper radiation corresponding to about 35 2) This is the part of the experimental pattern that contains information about the crystal packing, but no information about individual bond lengths or valence angles. The final coordinates of the H atoms were obtained by energy-minimizing the positions of the H atoms with the non-H atoms and the unit cell kept fixed, the coordinates of the H atoms in the accompanying CIF in the supporting information reflect nuclear positions rather than maxima in the electron density
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