Dynamic Nuclear Polarization NMR Spectroscopy of Microcrystalline Solids

This is a very interesting article, since it demonstrates how DNP can be tremendously helpful for the characterization of small molecules (e.g. quality control of pharmaceutical formulas). Here, DNP-enhanced 13C-13C correlation spectra are presented that are recorded using unlabeled material and the glass-forming solvents were carefully chosen so the analyte remains unaltered.

Rossini, A.J., et al., Dynamic Nuclear Polarization NMR Spectroscopy of Microcrystalline Solids. J. Am. Chem. Soc., 2012.

http://pubs.acs.org/doi/abs/10.1021/ja308135r

Dynamic nuclear polarization (DNP) solid-state NMR has been applied to powdered microcrystalline solids to obtain sensitivity enhancements on the order of 100. Glucose, sulfathiazole, and paracetamol were impregnated with bis-nitroxide biradical (bis-cyclohexyl-TEMPO-bisketal, bCTbK) solutions of organic solvents. The organic solvents were carefully chosen to be nonsolvents for the compounds, so that DNP-enhanced solid-state NMR spectra of the unaltered solids could be acquired. A theoretical model is presented that illustrates that for externally doped organic solids characterized by long spin?lattice relaxation times (T1(1H) > 200 s), 1H?1H spin diffusion can relay enhanced polarization over micrometer length scales yielding substantial DNP enhancements (ε). ε on the order of 60 are obtained for microcrystalline glucose and sulfathiazole at 9.4 T and with temperatures of ca. 105 K. The large gain in sensitivity enables the rapid acquisition of 13C-13C correlation spectra at natural isotopic abundance. It is anticipated that this will be a general method for enhancing the sensitivity of solid-state NMR experiments of organic solids.

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