Abstract
Nuclei with low gyromagnetic ratio (γ) present a serious sensitivity challenge for nulear magnetic resonance (NMR) spectroscopy. Recently, dynamic nuclear polarization (DNP) has shown great promise in overcoming this hurdle by indirect hyperpolarization (via 1H) of these low-γ nuclei. Here we show that at a magnetic field of 9.4 T and cryogenic temperature of about 110 K direct DNP of 89Y in a frozen solution of Y(NO3)3 can offer signal enhancements greater than 80 times using exogeneous trityl OX063 monoradical by satisfying the cross effect magic angle spinning (MAS) DNP mechanism. The large signal enhancement achieved permits 89Y NMR spectra of Y2O3 and Gd2O3-added Y2O3 materials to be obtained quickly (∼30 min), revealing a range of surface yttrium hydroxyl groups in addition to the two octahedral yttrium signals of the core. The results open up promises for the observation of low gyromagnetic ratio nuclei and the detection of corresponding surface and (sub-)surface sites.
Highlights
Nuclear magnetic resonance (NMR) spectroscopy is one of the most versatile analytical techniques used to understand the structure and dynamics of solid-state materials
Combining these techniques with dynamic nuclear polarization (DNP)[11−13] has introduced further, and dramatic, enhancements of the NMR signals from solid samples.[14−18] The DNP approach exploits the transfer of the high polarization of unpaired electrons (∼660 times larger than 1H), which are usually added to a sample in the form of stable free radicals[13] or transition metal ions,[19] to nuclei via suitable microwave irradiation[11,12] followed by their detection
Experiments were recorded with a 3.2 mm HXY triple resonance magic angle spinning (MAS) probe; for experiments with AMUPol[28], the probe was tuned to ν0(1H) = 400.321 MHz with the X channel tuned to ν0(13C) = 100.403 MHz and the Y channel tuned to ν0(89Y) = 19.700 MHz; for experiments with trityl, the probe was tuned to ν0(13C) = 100.725 MHz and ν0(89Y) = 16.672 MHz on the X and Y channels, respectively; these configurations correspond to the maximum signal enhancement for each nucleus observed in the MAS DNP magnetic field sweep profiles
Summary
Nuclear magnetic resonance (NMR) spectroscopy is one of the most versatile analytical techniques used to understand the structure and dynamics of solid-state materials. (a) 13C MAS DNP NMR spectra of Y(NO3)[3] in a solution of glycerol-d8/D2O/H2O in a 6:3:1 ratio (v/v) with μw on for trityl (blue) and AMUPol (orange) as the polarizing agent and with μw off (red).
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