Relationship between synthesis method-crystal structure-melting properties in cocrystals: the case of caffeine-citric acid

The influence of the crystal synthesis method on the crystallographic structure of caffeine-citric acid cocrystals was analyzed thanks to the synthesis of a new polymorphic form of the cocrystal. In order to compare the new form to the already known forms, the crystal structure of the new cocrystal (C8H10N4O2·C6H8O7) was solved by powder X-ray diffraction thanks to synchrotron experiments. The structure determination was performed using `GALLOP', a recently developed hybrid approach based on a local optimization with a particle swarm optimizer, particularly powerful when applied to the structure resolution of materials of pharmaceutical interest, compared to classical Monte-Carlo simulated annealing. The final structure was obtained through Rietveld refinement, and first-principles density functional theory (DFT) calculations were used to locate the H atoms. The symmetry is triclinic with the space group P-1 and contains one molecule of caffeine and one molecule of citric acid per asymmetric unit. The crystallographic structure of this cocrystal involves different hydrogen-bond associations compared to the already known structures. The analysis of these hydrogen bonds indicates that the cocrystal obtained here is less stable than the cocrystals already identified in the literature. This analysis is confirmed by the determination of the melting point of this cocrystal, which is lower than that of the previously known cocrystals.

Advanced perspectives in the EXPO software for solving crystal structures from powder diffraction data

Innovative computational and graphical tools have been implemented for advancing the crystal structure solution process from powder diffraction data using the EXPO software. These improve the capacity of EXPO to tackle challenging solution cases and make the structural study of microcrystalline materials more user friendly. The novel features concern the structure solution both in reciprocal space and in direct space. For reciprocal-space solution, the new capability to solve an unknown structure from a mixture composed of one unknown structure and one or more phases with known structures is particularly valuable. For direct-space structure solution, EXPO has been enhanced by parallelized simulated annealing with the aim of both reducing the execution time of the solution process and providing a successful result when several degrees of freedom must be varied. Many other new tools, functional for overcoming practical difficulties usually encountered during the solution process, have also been completed. In particular, a new tool for searching and importing structures from the Crystallography Open Database is effective for supporting the solution process in direct space. The advances in EXPO aim to provide software with improved reliability, expanded operation and greater speed.

An efficient treatment of ring conformations during molecular crystal structure determination from powder diffraction data

An effective and efficient method for dealing with ring systems during global optimisation-based crystal structure determination from powder diffraction data.