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

Four different structural models, which all fit the same X-ray powder pattern, were obtained in the structure determination of 4,11-di-fluoro-quinacridone (CHNOF) from unindexed X-ray powder data by a global fit. The models differ in their lattice parameters, space groups, , ', 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.

Structure determination from unindexed powder data from scratch by a global optimization approach using pattern comparison based on cross-correlation functions.

A method of ab initio crystal structure determination from powder diffraction data for organic and metal-organic compounds, which does not require prior indexing of the powder pattern, has been developed. Only a reasonable molecular geometry is required, needing knowledge of neither unit-cell parameters nor space group. The structures are solved from scratch by a global fit to the powder data using the new program FIDEL-GO (`FIt with DEviating Lattice parameters - Global Optimization').

Structure determination of organic compounds by a fit to the pair distribution function from scratch without prior indexing.

A method for the crystal structure determination of organic compounds by a fit to the pair distribution function (PDF), without prior knowledge of lattice parameters and space group, has been developed. The method is called 'PDF-Global-Fit' and is implemented by extension of the program (fit with deviating lattice parameters). The structure solution is based on a global optimization approach starting from random structural models in selected space groups.

Comparison and evaluation of pair distribution functions, using a similarity measure based on cross-correlation functions.

An approach for the comparison of pair distribution functions (PDFs) has been developed using a similarity measure based on cross-correlation functions. The PDF is very sensitive to changes in the local structure, small deviations in the structure can cause large signal shifts and significant discrepancies between the PDFs. Therefore, a comparison based on pointwise differences ( values and difference curves) may lead to the assumption that the investigated PDFs as well as the corresponding structural models are not in agreement at all, whereas a careful visual inspection of the investigated structural models and corresponding PDFs may reveal a relatively good match.

Orientational disorder of monomethyl-quinacridone investigated by Rietveld refinement, structure refinement to the pair distribution function and lattice-energy minimizations.

The crystal structure of the organic pigment 2-monomethyl-quinacridone (Pigment Red 192, CHNO) was solved from X-ray powder diffraction data. The resulting average structure is described in space group P\overline 1, Z = 1 with the molecule on the inversion centre. The molecules are arranged in chains.

Absolute Configuration of Pharmaceutical Research Compounds Determined by X-ray Powder Diffraction.

The absolute configuration of active pharmaceutical ingredients (APIs) was determined by generating salts of the active pharmaceutical ingredient (API) with counterions of known chirality, and determining the crystal structures by X-ray powder diffraction. This approach avoids the (often tedious) growth of single crystals, and is successful with very limited quantities of material (less than 1 mg). The feasibility of the method is demonstrated on five examples, and its limitations are discussed as well.