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Crystal structure prediction of organic pigments: quinacridone as an example.

Panina N, Leusen FJ, Janssen FF, Verwer P, Meekes H, Vlieg E, Deroover G - J Appl Crystallogr (2007)

Bottom Line: After generation and energy minimization of the possible structures, their powder patterns were compared with the experimental ones.Three possible structures for the beta polymorph are proposed on the basis of X-ray powder patterns comparison.It is shown that the alpha structure in the Cambridge Structural Database is likely to be in error, and a new alpha structure is proposed.

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ABSTRACT
The structures of the alpha, beta and gamma polymorphs of quinacridone (Pigment Violet 19) were predicted using Polymorph Predictor software in combination with X-ray powder diffraction patterns of limited quality. After generation and energy minimization of the possible structures, their powder patterns were compared with the experimental ones. On this basis, candidate structures for the polymorphs were chosen from the list of all structures. Rietveld refinement was used to validate the choice of structures. The predicted structure of the gamma polymorph is in accordance with the experimental structure published previously. Three possible structures for the beta polymorph are proposed on the basis of X-ray powder patterns comparison. It is shown that the alpha structure in the Cambridge Structural Database is likely to be in error, and a new alpha structure is proposed. The present work demonstrates a method to obtain crystal structures of industrially important pigments when only a low-quality X-ray powder diffraction pattern is available.

No MeSH data available.


Comparison of powder patterns of γ-QA: (a) experimental powder pattern, (b) calculated from experimental structure QNACRD04, (c) calculated from QNACRD04 after minimization in the force field, (d) calculated from structure 2, (e) calculated from structure 2 after Rietveld refinement (peak profiles not fitted for clarity).
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fig12: Comparison of powder patterns of γ-QA: (a) experimental powder pattern, (b) calculated from experimental structure QNACRD04, (c) calculated from QNACRD04 after minimization in the force field, (d) calculated from structure 2, (e) calculated from structure 2 after Rietveld refinement (peak profiles not fitted for clarity).

Mentions: To get an impression of the deviation from the experimental pattern, it is interesting to compare powder patterns for γ-QA. Fig. 12 ▶ shows the experimental powder pattern together with four calculated ones, namely calculated from the CSD structure QNACRD04 before and after energy minimization in the force field, the powder pattern of structure 2 in the list of predicted structures (minimized), and that of structure 2 after Rietveld refinement against the experimental data. In this figure it is remarkable that the patterns (c) and (d) are identical. This means that the structure predicted by the PP is the same as the experimental structure minimized using the same force field and, therefore, it is probable that these two structures are also identical.


Crystal structure prediction of organic pigments: quinacridone as an example.

Panina N, Leusen FJ, Janssen FF, Verwer P, Meekes H, Vlieg E, Deroover G - J Appl Crystallogr (2007)

Comparison of powder patterns of γ-QA: (a) experimental powder pattern, (b) calculated from experimental structure QNACRD04, (c) calculated from QNACRD04 after minimization in the force field, (d) calculated from structure 2, (e) calculated from structure 2 after Rietveld refinement (peak profiles not fitted for clarity).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2483485&req=5

fig12: Comparison of powder patterns of γ-QA: (a) experimental powder pattern, (b) calculated from experimental structure QNACRD04, (c) calculated from QNACRD04 after minimization in the force field, (d) calculated from structure 2, (e) calculated from structure 2 after Rietveld refinement (peak profiles not fitted for clarity).
Mentions: To get an impression of the deviation from the experimental pattern, it is interesting to compare powder patterns for γ-QA. Fig. 12 ▶ shows the experimental powder pattern together with four calculated ones, namely calculated from the CSD structure QNACRD04 before and after energy minimization in the force field, the powder pattern of structure 2 in the list of predicted structures (minimized), and that of structure 2 after Rietveld refinement against the experimental data. In this figure it is remarkable that the patterns (c) and (d) are identical. This means that the structure predicted by the PP is the same as the experimental structure minimized using the same force field and, therefore, it is probable that these two structures are also identical.

Bottom Line: After generation and energy minimization of the possible structures, their powder patterns were compared with the experimental ones.Three possible structures for the beta polymorph are proposed on the basis of X-ray powder patterns comparison.It is shown that the alpha structure in the Cambridge Structural Database is likely to be in error, and a new alpha structure is proposed.

View Article: PubMed Central - HTML - PubMed

ABSTRACT
The structures of the alpha, beta and gamma polymorphs of quinacridone (Pigment Violet 19) were predicted using Polymorph Predictor software in combination with X-ray powder diffraction patterns of limited quality. After generation and energy minimization of the possible structures, their powder patterns were compared with the experimental ones. On this basis, candidate structures for the polymorphs were chosen from the list of all structures. Rietveld refinement was used to validate the choice of structures. The predicted structure of the gamma polymorph is in accordance with the experimental structure published previously. Three possible structures for the beta polymorph are proposed on the basis of X-ray powder patterns comparison. It is shown that the alpha structure in the Cambridge Structural Database is likely to be in error, and a new alpha structure is proposed. The present work demonstrates a method to obtain crystal structures of industrially important pigments when only a low-quality X-ray powder diffraction pattern is available.

No MeSH data available.