Pigment Orange 5: crystal structure determination from a non-indexed X-ray powder diagram

Schmidt, Martin; Buchsbaum, Christian; Schnorr, Jan M.; Hofmann, Detlef W. M.; Ermrich, M.

doi:10.1524/zkri.2007.222.1.30

Cited by 10 publications

(3 citation statements)

References 13 publications

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“…In solution, this isomerization has been found to give a dynamic equilibrium of both forms with varying ratios, depending on the substitutions on the aromatic cores. , Crystallization can predominantly stabilize one form, giving either azo or hydrazone solid pigment. In the last four decades, there have been overwhelming crystallographic evidence that β-naphthol reds tend to crystallize as hydrazones. − The same has also been found for numerous “lake pigments,” which are β-naphthol reds that feature sulfonate and/or carboxylate groups, and “naphthol AS pigments” − that are various anilides of 3-hydroxy-2-carboxynaphthalene. There is a limited number of β-naphthol reds that have been shown to exhibit a temperature-dependent equilibrium of azo and hydrazone forms in the solid state. − Following these extensive studies, there have been efforts to correctly designate β-naphthol reds as hydrazones and not “azo pigments.”…”

Section: Introductionmentioning

confidence: 77%

Azo or Not: Continuing the Crystallographic Investigations of β-Naphthol Reds

Heaney

Unruh

Runčevski

2022

Crystal Growth & Design

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β-Naphthol reds are a group of widely used pigments with prominent historical, commercial, and cultural significance. In industry, and especially within the art and heritage community, they are known as azo pigments. However, β-naphthols, very often, are not azo pigments. Because of enol/keto tautomerization of the 1-arylhydrazone-2-naphthol skeleton, these pigments oftentimes crystallize as hydrazones (keto). Therefore, proper characterization is necessary for understanding their intrinsic physicochemical properties and chemical reactivity in the solid state, as well as their stability and lightfastness. Crystallographic studies have indicated that β-naphthol reds tend to adopt the hydrazone form in the solid state. Here, we continue these structural investigations, and we focused on two prominent β-naphthol reds, pigment red 40 (PR40) and pigment red 4 (PR4). Using single-crystal X-ray diffraction, we provide decisive proof that both of these pigments are keto/hydrazones in the solid state. Therefore, the frequent yet erroneous designation as azo pigments should be avoided. To confirm the bulk structure, we performed powder diffraction experiments, followed by Rietveld refinement. We complemented the diffraction experiments with spectroscopic (IR, Raman, UV–vis) and thermal (TGA, DSC) analyses. Furthermore, we studied the lightfastness of both chromophores in solution and the solid state. While the solid-state pigments were stable over the course of the experiment, UV irradiation of solutions resulted in degradation, which was studied by chromatographic and mass-spectrometry techniques. We hope that this research will bring to light the necessity of proper solid-state characterization of β-naphthol reds, as well as pigments as a whole.

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Section: Introductionmentioning

confidence: 77%

Azo or Not: Continuing the Crystallographic Investigations of β-Naphthol Reds

Heaney

Unruh

Runčevski

2022

Crystal Growth & Design

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“…Very few organic structures have been solved from scratch by crystal structure prediction with a global optimization of the lattice energy using force-field methods (e.g. Schmidt & Dinnebier, 1999;Schmidt et al, 2005;Hammer et al, 2011) or quantum mechanical methods (Day et al, 2006;Zhu et al, 2016;Askin et al, 2019;Bhardwaj et al, 2019) or using statistical potentials (Hofmann & Kuleshova, 2005;Schmidt et al, 2007). According to our experience, most predicted structures which have favourable lattice energies exhibit very different simulated powder patterns, and there is usually only one structure which simultaneously has a good energy and fits the powder data well.…”

Section: Introduction 1ambiguous Crystal Structure Solutionsmentioning

confidence: 99%

Ambiguous structure determination from powder data: four different structural models of 4,11-difluoroquinacridone with similar X-ray powder patterns, fit to the PDF, SSNMR and DFT-D

Schlesinger

Fitterer

Buchsbaum

et al. 2022

IUCrJ

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Four different structural models, which all fit the same X-ray powder pattern, were obtained in the structure determination of 4,11-difluoroquinacridone (C20H10N2O2F2) from unindexed X-ray powder data by a global fit. The models differ in their lattice parameters, space groups, Z, Z′, molecular packing and hydrogen bond patterns. The molecules form a criss-cross pattern in models A and B, a layer structure built from chains in model C and a criss-cross arrangement of dimers in model D. Nevertheless, all models give a good Rietveld fit to the experimental powder pattern with acceptable R-values. All molecular geometries are reliable, except for model D, which is slightly distorted. All structures are crystallochemically plausible, concerning density, hydrogen bonds, intermolecular distances etc. All models passed the checkCIF test without major problems; only in model A a missed symmetry was detected. All structures could have probably been published, although 3 of the 4 structures were wrong. The investigation, which of the four structures is actually the correct one, was challenging. Six methods were used: (1) Rietveld refinements, (2) fit of the crystal structures to the pair distribution function (PDF) including the refinement of lattice parameters and atomic coordinates, (3) evaluation of the colour, (4) lattice-energy minimizations with force fields, (5) lattice-energy minimizations by two dispersion-corrected density functional theory methods, and (6) multinuclear CPMAS solid-state NMR spectroscopy (1H, 13C, 19F) including the comparison of calculated and experimental chemical shifts. All in all, model B (perhaps with some disorder) can probably be considered to be the correct one. This work shows that a structure determination from limited-quality powder data may result in totally different structural models, which all may be correct or wrong, even if they are chemically sensible and give a good Rietveld refinement. Additionally, the work is an excellent example that the refinement of an organic crystal structure can be successfully performed by a fit to the PDF, and the combination of computed and experimental solid-state NMR chemical shifts can provide further information for the selection of the most reliable structure among several possibilities.

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“…Even a separate patent covering this new crystal structure can be applied for. In the dye industry the situation is comparable [4,5]. For these reasons a good knowledge about possible polymorphs is essential.…”

Section: Introductionmentioning

confidence: 99%