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Issue 22, 2016
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4-Aminoquinaldine monohydrate polymorphism: prediction and impurity aided discovery of a difficult to access stable form

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Abstract

Crystal structure prediction studies indicated the existence of an unknown high density monohydrate structure (Hy1B°) as the global energy minimum for 4-aminoquinaldine (4-AQ). We thus performed an interdisciplinary experimental and computational study elucidating the crystal structures, solid form inter-relationships, and kinetic and thermodynamic stabilities of the stable anhydrate (AH I°), the kinetic monohydrate (Hy1A) and this novel monohydrate polymorph (Hy1B°) of 4-AQ. The crystal structure of Hy1B° was determined by combining laboratory powder X-ray diffraction data and ab initio calculations. Dehydration studies with differential scanning calorimetry and solubility measurements confirmed the result of the lattice energy calculations, which identified Hy1B° as the thermodynamically most stable hydrate form. At 25 °C the equilibrium of the 4-AQ hydrate/anhydrate system was observed at an aw (water activity) of 0.14. The finding of Hy1B° was complicated by the fact that the metastable but kinetically stable Hy1A shows a higher nucleation and growth rate. The presence of an impurity in an available 4-AQ sample facilitated the nucleation of Hy1B°, whose crystallisation is favored under hydrothermal conditions. The value of combining experimental with theoretical studies in hydrate screening and characterisation, as well as the reasons for hydrate formation in 4-AQ, are discussed.

Graphical abstract: 4-Aminoquinaldine monohydrate polymorphism: prediction and impurity aided discovery of a difficult to access stable form

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Supplementary files

Article information


Submitted
02 Sep 2015
Accepted
09 Oct 2015
First published
15 Oct 2015

CrystEngComm, 2016,18, 4053-4067
Article type
Paper

4-Aminoquinaldine monohydrate polymorphism: prediction and impurity aided discovery of a difficult to access stable form

D. E. Braun, H. Oberacher, K. Arnhard, M. Orlova and U. J. Griesser, CrystEngComm, 2016, 18, 4053
DOI: 10.1039/C5CE01758K

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