Abstract
Despite the abundance of hydrates, their multifaceted nature and hydration/dehydration behaviour is still not fully understood. For the example of orotic acid monohydrate and its lithium and magnesium hydrate salts, we show how NMR crystallography, namely a combination of solid-state NMR with a focus here on 1H magic angle spinning (MAS) NMR experiments and first-principles DFT GIPAW (gauge-including projector augmented wave) calculations, can play a valuable role in the characterization of hydrate systems. Starting from lithium orotate monohydrate, a rigid system with a limited number of tightly bound water molecules, the general feasibility of this approach was demonstrated. Moving onto more complex hydrate structures, mobility in the orotic acid monohydrate was observed, while for the most complex hydrate, magnesium orotate octahydrate, a loss of associated water molecules was observed after an overnight MAS NMR experiment. A combined study by experimental MAS NMR, powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) revealed changes after vacuum drying as well as after storage of a vacuum dried sample under ambient conditions. Specifically, TGA showed the vacuum dried sample to correspond to a dihydrate, for which no structure has yet been determined by single-crystal X-ray diffraction. An NMR crystallography analysis showed that a combination of putative symmetric and asymmetric dihydrate structures explains the observed changes in the experimental MAS NMR spectra.
Highlights
Hydrates are a very versatile but complex group of compounds
For the example of orotic acid monohydrate and its lithium and magnesium hydrate salts, we show how NMR crystallography, namely a combination of solid-state NMR with a focus here on 1H magic angle spinning (MAS) NMR experiments and first-principles DFT GIPAW calculations, can play a valuable role in the characterization of hydrate systems
Lithium orotate forms a contact ion pair (CIP), while in 3, the magnesium atom is separated from the counter ion by a shell of water molecules
Summary
Hydrates are a very versatile but complex group of compounds. At various stages of manufacturing, processing or storing of chemical compounds, the abundance of water facilitates the occurrence of these molecular adducts. Orotic acid monohydrate as well as the corresponding lithium and magnesium orotate hydrates (Scheme 1) are analysed by a combination of solid-state NMR together with GIPAW (CASTEP)[24,25,26] calculations, powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) These compounds have been tested for very diverse applications such as the treatment of cardiovascular illnesses[27,28,29,30] or bipolar disorder,[31,32] diagnosis of malfunctions in the pyrimidine metabolism[33,34] and use as a nucleating agent for the crystallization of biodegradable polymers.[35,36] they represent a set of structures with, in principle, known single crystal X-ray structures (see Fig. 1), in which each individual compound belongs to a different class of hydrates. This NMR crystallography study reveals complexities in structure and dynamics that go beyond the static view of the available crystal structures
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