Solid-state NMR is a very powerful technique that can provide atomic-level structural details on solid samples independently of their long-range order. As a consequence, solid-state NMR is suitable to characterize samples forming sub-micrometric crystallites (i.e. powders) or amorphous aggregates, making it highly complementary to X-ray diffraction. The main drawback of NMR is its inherently low sensitivity. In the last decade, the combination of solid-state NMR with dynamic nuclear polarization has alleviated this issue and opened a plethora of new potential experimental possibilities. In our laboratory, we capitalize on the tremendous NMR sensitivity enhancement provided by DNP to obtain unique insights into the structure and morphology of molecular crystals and their crystallization from solution. In this communication, I will first present some of our recent results on the use of solid-state NMR for structural determination of molecular crystals existing as powders. I will show that three-bond 13C-13C J couplings accessed by solid-state NMR on 13C-enriched organic crystals can give access to the molecular conformation in the solid state, with great potential for reducing the computational cost of crystal structure prediction methods. Then, I will focus on how solid-state NMR, combined with DNP, can be used to investigate the mechanistic aspects of crystallization processes occurring ex-situ (i.e. outside the spectrometer). I will discuss current approaches and recent developments allowing to detect and characterize transient, metastable phases formed at the early stages of crystallization through the use of specific DNP polarizing agents.

Solid-state NMR and DNP for the structural investigation of organic powders and crystallization processes

ACS Spring Meeting 2021

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