Abstract
The moisture- and temperature dependent stabilities and interrelation pathways of the practically relevant solid forms of o-phenanthroline HCl (1) and neocuproine HCl (2) were investigated using thermal analytical techniques (HSM, DSC and TGA) and gravimetric moisture sorption/desorption studies. The experimental stability data were correlated with the structural changes observed upon dehydration and the pairwise interaction and lattice energies calculated. For 1 the monohydrate was identified as the only stable form under conditions of RH typically found during production and storage, but at RH values >80% deliquescence occurs. The second compound, 2, forms an anhydrate and two different hydrates, mono- (2-Hy1) and trihydrate (2-Hy3). The 2-Hy1 structure was solved from SCXRD data and the anhydrate structure derived from a combination of PXRD and CSP. Depending on the environmental conditions (moisture) either 2-Hy1 or 2-Hy3 is the most sable solid form of 2 at RT. The monohydrates 1-Hy1 and 2-Hy1 show a high enthalpic stabilization (≥20 kJ mol−1) relative to the anhydrates. The anhydrates are unstable at ambient conditions and readily transform to the monohydrates even in the presence of traces of moisture. This study demonstrates how the right combination of experiment and theory can unravel the properties and interconversion pathways of solid forms.
Highlights
IntroductionOrganic molecules often occur in several crystalline forms, such as polymorphs (same chemical composition), hydrates (water-adducts), and solvates (organic solvent-adducts)
Organic molecules often occur in several crystalline forms, such as polymorphs, hydrates, and solvates
The two chemically related HCl salts have a high affinity towards water, in other words under conditions of RH typically found during production and storage the hydrates are the stable forms
Summary
Organic molecules often occur in several crystalline forms, such as polymorphs (same chemical composition), hydrates (water-adducts), and solvates (organic solvent-adducts). These different solid forms are important research targets, in the pharmaceutical and other fine-chemical industry, because they commonly have different physical or chemical solid-state properties, such as solubility, dissolution rate, density, chemical and physical stability which may have considerable consequences for manufacturing processes and the performance of final products [1,2,3]. Depending on on the the water wateractivity activityand andthe thetemperature temperaturea ahydrate hydrate can most stable form. In-depth investigations and of such interchanges for both practical and regulatory reasons. In-depth investigations a sound understanding of the reasons forfor hydrate formation and a sound understanding of the reasons hydrate formationisisalso alsovery veryimportant importantto to support support the the progresses in computational efforts aiming at reliable predictions of hydrate formation and stability
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