In situ solid-state NMR spectroscopy is exploited to monitor the structural evolution of a glycine/water glass phase formed on flash cooling an aqueous solution of glycine, with a range of modern solid-state NMR methods applied to elucidate structural properties of the solid phases present. The glycine/water glass is shown to crystallize into an intermediate phase, which then transforms to the β polymorph of glycine. Our in situ NMR results fully corroborate the identity of the intermediate crystalline phase as glycine dihydrate, which was first proposed only very recently.
Establishing mechanistic
understanding of crystallization processes
at the molecular level is challenging, as it requires both the detection
of transient solid phases and monitoring the evolution of both liquid
and solid phases as a function of time. Here, we demonstrate the application
of dynamic nuclear polarization (DNP) enhanced NMR spectroscopy to
study crystallization under nanoscopic confinement, revealing a viable
approach to interrogate different stages of crystallization processes.
We focus on crystallization of glycine within the nanometric pores
(7–8 nm) of a tailored mesoporous SBA-15 silica material with
wall-embedded TEMPO radicals. The results show that the early stages
of crystallization, characterized by the transition from the solution
phase to the first crystalline phase, are straightforwardly observed
using this experimental strategy. Importantly, the NMR sensitivity
enhancement provided by DNP allows the detection of intermediate phases
that would not be observable using standard solid-state NMR experiments.
Our results also show that the metastable β polymorph of glycine,
which has only transient existence under bulk crystallization conditions,
remains trapped within the pores of the mesoporous SBA-15 silica material
for more than 200 days.
A method based on highly concentrated radical solutions is investigated for the suppression of the NMR signals arising from solvents that are usually used for dynamic nuclear polarization experiments. The presented method is suitable in the case of powders, which are impregnated with a radical-containing solution. It is also demonstrated that the intensity and the resolution of the signals due to the sample of interest is not affected by the high concentration of radicals. The method proposed here is therefore valuable when sensitivity is of the utmost importance, namely samples at natural isotopic abundance.
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