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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.

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.


Structure of β3-QA minimized in the force field (red) and after the Rietveld refinement (blue).
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fig9: Structure of β3-QA minimized in the force field (red) and after the Rietveld refinement (blue).

Mentions: Rietveld refinement of the β1 and β2 structures against the experimental powder pattern resulted in both cases in a reasonable fit with R values of 13.0 and 12.6%, respectively. The difference can be noted at 15° in Fig. 5 ▶, where β1 has two peaks and β2 only one. The corresponding peak in the experimental pattern is broad and makes it impossible to distinguish between these two structures. It is notable that both refined structures are very close in the energy density plot (Fig. 2b ▶). Both structures of β3 undergo cell compression during the Rietveld refinement due to the shortening of the long axis. The energy minimization performed between the Rietveld steps forces molecules to leave the nearly parallel arrangement of stacks (see Fig. 9 ▶). The density is nevertheless hardly affected (Fig. 2b ▶). The refinement converges at R values of 19.7% and 20.6% for the structures 13 and 14, respectively, and leads to nearly the same structure in both cases. Therefore, only the crystallographic data for the refined structure 13 are listed in Table 2 ▶.


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)

Structure of β3-QA minimized in the force field (red) and after the Rietveld refinement (blue).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig9: Structure of β3-QA minimized in the force field (red) and after the Rietveld refinement (blue).
Mentions: Rietveld refinement of the β1 and β2 structures against the experimental powder pattern resulted in both cases in a reasonable fit with R values of 13.0 and 12.6%, respectively. The difference can be noted at 15° in Fig. 5 ▶, where β1 has two peaks and β2 only one. The corresponding peak in the experimental pattern is broad and makes it impossible to distinguish between these two structures. It is notable that both refined structures are very close in the energy density plot (Fig. 2b ▶). Both structures of β3 undergo cell compression during the Rietveld refinement due to the shortening of the long axis. The energy minimization performed between the Rietveld steps forces molecules to leave the nearly parallel arrangement of stacks (see Fig. 9 ▶). The density is nevertheless hardly affected (Fig. 2b ▶). The refinement converges at R values of 19.7% and 20.6% for the structures 13 and 14, respectively, and leads to nearly the same structure in both cases. Therefore, only the crystallographic data for the refined structure 13 are listed in Table 2 ▶.

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.