It is notable that the tetarto-hydrate forms a tetragonal structure with water in channels, a framework that although stabilized by water, is found as a higher energy structure on a computationally generated crystal energy landscape, which has the anhydrate crystal structure as the most stable form. Thus, a combination of slurry experiments, X-ray diffraction, spectroscopy, moisture de sorption, and thermo-analytical methods with the computationally generated crystal energy landscape and lattice energy calculations provides a consistent picture of the finely balanced hydration behavior of pyrogallol.
In addition, two monotropically related dimethyl sulfoxide monosolvates were found in the accompanying solid form screen. The structural transformation between anhydrous and hydrated pyrogallol, the only two practically relevant forms emerging from a solid form screen, has been unraveled with complementary experimental techniques moisture sorption analysis, thermal analysis, X-ray diffraction, and water activity measurements and crystal energy landscape calculations.
Figure 1. Pyrogallol PG conformers with atom numbering used throughout this study. Intramolecular hydrogen bonds are indicated with red dashed lines. Figure 2. Figure 3. Figure 4. Figure 5. Packing diagrams and hydrogen bonding of pyrogallol anhydrate viewed along , with the C 1 1 6 chain hydrogen bonding direction horizontal. Figure 6. Packing diagrams of pyrogallol tetarto-hydrate viewed along a  and b , with  water channels vertical.
Crystallographically unique molecules are colored differently. Positions A and B mark the two water sites site occupancy factor of 0. Figure 7.
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Figure 8. Anhydrous pyrogallol was used as a starting phase; the residual phase, after stirring for ten days, was determined with PXRD. Figure 9. DSC thermograms of the tetarto-hydrate in 2 a three pin-holed pan and a heating rate of 2. Figure Each symbol denotes a crystal structure, which is a lattice energy minimum classified by the most extensive common-packing motif based on the hydrogen bonding shown in b. Tie lines have been added to show the changes in relative ordering.
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The numbers labeling the symbols in a identify the crystal structures by stability order using the PCM model Table S9a of the Supporting Information. Only selected symmetry operations are drawn in b.
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Overlay of the experimental crystal structure of pyrogallol anhydrate colored by element and the most stable computed structure after full relaxation of the cell and atomic coordinates with DFT-D green. Illustration of the packing similarities and hydrogen bonding motifs of common building blocks in lowest energy structures on the pyrogallol crystal energy landscape Figure Water molecules were added, after symmetry reducing the anhydrate structure, in different orientations I and II in close proximity to the pyrogallol hydroxyl groups section 2.
Energy values correspond to the intermolecular energy contribution of the water molecule to the tetarto-hydrate lattice energy. Overlay of the experimental crystal structure of pyrogallol tetarto-hydrate 27 colored by element and computationally derived hydrate structure green. The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the manuscript. Pantelides and Dr. Conformer C structures were found to be too high in lattice energy, and therefore, not included in the additional search. View Author Information.
Doris E. Cite this: Cryst.
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Growth Des. ACS AuthorChoice. Article Views Altmetric -. Citations PDF 8 MB. Abstract High Resolution Image.
Synopsis The structural transformation between anhydrous and hydrated pyrogallol, the only two practically relevant forms emerging from a solid form screen, has been unraveled with complementary experimental techniques moisture sorption analysis, thermal analysis, X-ray diffraction, and water activity measurements and crystal energy landscape calculations.
Understanding the diversity of crystalline forms polymorphs, hydrates, and solvates is important in the pharmaceutical and other fine-chemical industries. Both kinetic and thermodynamic factors determine which phase results. On the thermodynamic side, contrasting the crystal energy landscape of a molecule 4 i.
Thus, systematic changes in solvent including mixtures, water activity , temperature, and rate of change of supersaturation need to be considered in solid form screens, with many other variables being possible. Solvates are formed when the solvent of crystallization becomes part of the crystal lattice, 2 with the largest number of solvates containing water hydrates. However, though hydrates are usually among the first solid state forms that are discovered in polymorph screens, a clear picture of their thermodynamic and kinetic stability is rarely elaborated.
Our strategy to achieve a better understanding of hydrates aims at comprehensive analytical investigations of model hydrates involving computational approaches to connect structural features with relevant properties, particularly stability. One of the selected model compounds is pyrogallol 1,2,3-trihydroxybenzene, pyrogallic acid, PG, Figure 1 , a small organic molecule, used in analytical chemistry as a reagent for antimony and bismuth, as a reducing agent for gold, silver, and mercury salts, and for oxygen absorption in gas analysis.
It was used in photography, for dyeing furs, hair, etc. Recently, the crystal structures of the AH and HY0.
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High Resolution Image. Our study aimed at a comprehensive qualitative and quantitative understanding of the structural, thermodynamic, and kinetic stability of the two practically most important crystalline forms of PG. This was confirmed with an extensive solid form screen, which resulted in two additional solvate forms. The experimental results were complemented with computational modeling i.
For the solvent screen, a set of 29 solvents was chosen, which were all of analytical quality.
The solvent screen included evaporation, cooling crystallization, antisolvent precipitation, and liquid-assisted grinding experiments. For liquid-assisted grinding experiments, anhydrous pyrogallol and few drops of each solvent were ground in a Retsch grinding mill MM for 7. Suitable crystals were obtained from slow evaporation experiments from dimethyl sulfoxide. All non-H atoms were refined anisotropically. For further details see ref The sample was loaded in a rotating 1. Data was collected at room temperature using a variable count time scheme 37, 38 Table S5 of the Supporting Information.
The data were background subtracted, and Pawley refinement 41 was used to extract the intensities and their correlations. Simulated annealing was used to optimize the hydrate model against the diffraction data set reflections in direct space. The structure was solved using simulated annealing runs of 2. Each of the two PG molecules was allowed 6 external degrees of freedom, and for the water molecule only oxygen was included i.
As expected, the improvement R wp came at the expense of some chemical sense e. Approximately mg of the AH and mg of the HY0. Excess of pyrogallol AH and HY0. Samples were withdrawn and filtered, and the resulting phase was determined using powder X-ray diffraction and thermogravimetric analysis. For hot-stage thermomicroscopic investigations HTM , a Reichert Thermovar polarization microscope equipped with a Kofler hot-stage Reichert, A was used. Dry nitrogen was used as the purge gas purge: 20 mL min —1.
Heating rates of 2. The instrument was calibrated for temperature with pure benzophenone mp An approximately 3—5 mg sample was weighed into a platinum pan. A two-point calibration of the temperature was performed with ferromagnetic materials Alumel and Ni, Curie-point standards, Perkin-Elmer. An additional crystal structures containing conformer A 49 were generated in each of the possible hydrate space groups derived from indexing see X-ray Diffractometry and the literature 25 [i.
The result is the PCM crystal energy landscape. The sensitivity of the relative energies of the structures to the modeling assumptions was investigated by using other methods of evaluating lattice energies. PIXEL calculations 32, 66, 67 were also carried out on these low-energy structures to estimate the repulsive E R , dispersion E D , electrostatic Coulombic, E C , and induction polarization E P contributions to the intermolecular lattice energy, and the subdivision of intermolecular lattice energy into contributions from individual pairs of molecules within a crystal.
Both the PCM and DFT-D optimized crystal structures were used to test sensitivity of the energies to small differences in the crystal structure 68 section 2. The electron density was described using medium cube settings and a step size of 0.