Isomorphism, Disorder, and Hydration in the Crystal Structures of Racemic and Single-Enantiomer Carvedilol Phosphate

Citations Over TimeTop 18% of 2010 papers

Abstract

Understanding the crystalline structure of racemic carvedilol phosphate hemihydrate presents several challenges that were overcome using a combination of single-crystal X-ray diffraction, solid-state NMR (SSNMR), and other analytical techniques. Initial attempts to obtain a crystal structure were hampered by difficulties with twinning and problematic disorder in the final refinements. Multinuclear SSNMR analysis localized the disorder to portions of the molecule near the chiral center. As a result, single-enantiomer carvedilol phosphate was prepared and was found to crystallize in a phase that was isomorphous with the racemate, while SSNMR spectra of the single enantiomers did not contain the disorder observed in the racemate. The single-crystal X-ray structure of the (R)-enantiomer was solved and used as a starting point to successfully progress the solution of the disordered racemic crystal structure. Thermal analysis and construction of a phase diagram, along with crystallographic and spectroscopic analysis, found the crystal structure of the racemate to be a solid solution of (R)- and (S)-enantiomers, with the conformation of the molecule adjusting to fit. The crystal structures show the stoichiometry of the both the racemate and (R)-enantiomer to be a hemihydrate. The phase isomorphically dehydrates below relative humidity values of 1% and above temperatures of 125 °C as assessed by water vapor sorption studies, powder X-ray diffraction, and SSNMR. Single-crystal diffraction detected significant changes in the unit cell dimensions as the phase dehydrated, which was related to the visual appearance of opacity in a single crystal of the (R)-enantiomer. The mechanism of water incorporation was further probed spectroscopically via exchange with deuterium, 17O-, and 18O-labeled water; the results suggest that dehydration and rehydration likely proceed via narrow tunnels in the crystal structure, combined with the formation of fissures in the crystal. 2H SSNMR experiments showed that the water does not engage in solid-state jump motion even at higher temperatures.

Related Papers

• → Alteration of Polymorphic Selectivity through Different Crystallization Mechanisms Occurring in the Same Crystallization Solution(2007)35 cited
• → Effect of the weather conditions during solution preparation on lysozyme crystallization(2017)1 cited
• New progress in research on crystallization technology(2001)
• Study on the Crystallization Process of Antioxidant BHT(2005)