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Is able to PatternMatch generate physical property information from XRPD patterns, thus maximising the amount of useful information that can be obtained from routine screening data.
Properties such as crystal unit cell volume, morphology and density are obtained from the XRPD data of each form. The unit cell volume is obtained from indexing results, density is calculated, and morphology is predicted using Shape (version 7.0; Shape is a program for drawing Explosional drawing the external morphology, or faces, of crystals) and the Bravais-Friedel-Donnay-Harker
( BFDH) model [15,16]. The BFDH model, which was initially developed for inorganic molecules, predicts crystal morphologies based on unit cell parameters and the symmetry, and, in general, provides a good first step in predicting the morphology of organic molecules. When the BFDH model fails to predict the morphology, it suggests that certain preferred anisotropic bonding exists, which is not governed by the unit cell and symmetry. Example For, this offers unique opportunities to modify the morphology of the crystalline form using solvents and additives.
Indexing is a process of extracting crystal unit cell parameters from XRPD data, from which a crystal structure can be solved. However, solving crystal structures of complex organic compounds was traditionally limited to high resolution XRPD data [17,18], due to limited information available from conventional diffractometers. However, the PatternMatch program allows for highly successful indexing of data obtained from common laboratory X-ray diffractometers, enabling structure solutions. An example is found in a recent publication, in which a new crystalline form of prasterone was indexed using PatternMatch and the crystal structure was determined [19].
The PatternMatch program was used to obtain unit cell data from the XRPD patterns of the five crystalline forms of buspirone hydrochloride, which yielded unit cell volumes and from which unoccupied molecular volumes were calculated. The PatternMatch program allows for automated volume-symmetry indexing using XRPD patterns. The simplicity of this option is that the resulting set of solutions that best describe the experimental data do not have to be exactly correct as the multiple potential solutions cluster around a certain volume (see Table 1 for Form C). The best solutions are those that describe all the measured peak positions within 0.02 ° 2θ and do not exhibit additional calculated peaks in the regions of the measured XRPD pattern where no peaks were observed. The comparisons of the measured powder pattern to indexed solutions are shown in Figure 3 for Forms A and C. The volume information for each crystalline form is then used to estimate the unoccupied volume (Table 2). Forms A, C and D show relatively small unoccupied volume indicating they are anhydrous and nonsolvated solid forms. The remaining solid Forms, B and E, however, show a significant unoccupied volume. Thermogravimetric analyses data confirmed that Forms B and E contain volatiles, and Forms A, C and D are anhydrous. Densities of the anhydrous forms are calculated using the unit cell volumes and molecular weight of the buspirone hydrochloride. For the solvated forms (Forms B and E), they were assumed as 1,2-dichloroethane and chloroform solvates, respectively, based on the solvents they were obtained from, and the appropriate molecular weights that were used for the density calculations (Table 2 ). The density rule allows the prediction that the less dense polymorph is metastable [20].
Therefore, the density data of the three anhydrous forms, A, C and D yield the following thermodynamic stability order (the most to the least stable): A> C> D. How does this prediction compare to the experimental data? Form A and Form C match P188 and P203, respectively, of reference [108] and [10], based on the melting temperature by differential scanning calorimetry analyses. According to the literature [10], the two crystalline forms are enantiotropically related, where Form C is the thermodynamically less stable form below the transition temperature (95 ° C), and the more stable form above the transition temperature. Therefore, Form A is the thermodynamically more stable form at ambient conditions up to 95 C °, which is consistent with the predictions based on the indexed information using PatternMatch.
The best indexing solution is selected and refined by generating electron density maps. The electron density is viewed in terms of effective nodes or atom sites, which allows the identification of gross features within the average unit cell. The effective resolution of these maps is ~ 2.0 - 2.5 Å; however, for the more crystalline material, it is often possible to identify the molecule within the unit cell. Electron density maps help to predict the properties, such as morphology, compressibility and physical stability, more accurately. The electron density map generated for Form A is shown in Figure 4. The electron density map of Form A is in good agreement with the published single crystal structure data [21]. The electron density map of Form B exhibits spaces which are consistent with the initial assessment that this may be a solvate based on the calculated volume. The final test for the correctness of an indexing solution is the ability to pack the molecule into the crystal unit cell and approximate the peak intensities of the experimental XRPD data using DASH software (Cambridge Crystallographic Data Centre). The resulting crystal structures were loaded into the Rietveld program MAUD for final refinement, and the calculated powder patterns were compared to the experimental data, which confirmed the correctness of the indexing solution. In addition, the morphology of the two most relevant forms, Forms A and C, were determined. Shape (version 7.0) correctly predicted plate-like crystal morphology for Form A, and needle-like crystal morphology for Form C (Figure 5).
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