Development of DNP-Enhanced High-Resolution Solid-State NMR System for the Characterization of the Surface Structure of Polymer Materials

Abstract

A dynamic nuclear polarization (DNP)-enhanced cross-polarization/magic-angle spinning (DNP/CP/MAS) NMR system has been developed by combining a 200 MHz Chemagnetics CMX-200 spectrometer operating at 4.7 T with a high-power 131.5 GHz Gyrotron FU CW IV. The 30 W sub-THz wave generated in a long pulse TE ð1Þ 41 mode with a frequency of 5 Hz was successfully transmitted to the modified Doty Scientific lowtemperature CP/MAS probe through copper smooth-wall circular waveguides. Since serious RF noises on NMR signals by arcing in the electric circuit of the probe and undesired sample heating were induced by the continuous sub-THz wave pulse irradiation with higher powers, the on-off sub-THz wave pulse irradiation synchronized with the NMR detection was developed and the appropriate setting of the irradiation time and the cooling time corresponding to the non-irradiation time was found to be very effective for the suppression of the arcing and the sample heating. The attainable maximum DNP enhancement was more than 30 folds for C1 13 C-enriched D-glucose dissolved in the frozen medium containing mono-radical 4-amino-TEMPO. The first DNP/CP/MAS 13 C NMR spectra of poly(methyl methacrylate) (PMMA) sub-micron particles were obtained at the dispersed state in the same frozen medium, indicating that DNP-enhanced 1 H spins effectively diffuse from the medium to the PMMA particles through their surface and are detected as high-resolution 13 C spectra in the surficial region to which the 1 H spins reach. On the basis of these results, the possibility of the DNP/CP/MAS NMR characterization of the surface structure of nanomaterials including polymer materials was discussed.

Solid state nuclear magnetic resonance with magic-angle spinning and dynamic nuclear polarization below 25K

“…Demonstrations by Griffin and coworkers [2] of large nuclear magnetic resonance (NMR) signal enhancements in frozen solutions at high magnetic fields and under magic-angle spinning (MAS) through dynamic nuclear polarization (DNP) have stimulated much interest in DNP for solid state NMR studies of biological [3–9] and non-biological [10–14] systems in recent years. A popular approach is to use continuous-wave gyrotron microwave sources, capable of producing many watts at frequencies above 200 GHz [15–18], and to operate at sample temperatures in the 80–100 K range [19, 20].…”

Recent Advances in Magic Angle Spinning‐Dynamic Nuclear Polarization Methodology

“…The technology has been commercialized by Bruker Biospin in collaboration with Communications & Power Industries, and complete gyrotron‐based MAS‐DNP spectrometers are commercially available for 400, 600, 800, and 900 MHz proton frequencies with gyrotrons operating at 263, 395, 527, and 593 GHz, respectively (Figure a) . In addition, several companies supply DNP‐grade gyrotrons for various home‐built instruments, and other gyrotron‐based systems have been built in collaboration with research groups specializing in high‐power sub‐THz technology …”

DNP NMR Studies of Crystalline Polymer Domains by Copolymerization with Nitroxide Radical Monomers

“…The sample target temperature was set to 100 K unless otherwise specified, but it should be noted that the microwave irradiation applied during the DNP experiments will cause some sample heating. 1 H and 13 C spectra were recorded using single-pulse and crosspolarization (CP) pulse sequences, respectively, with chemical shifts referenced to TMS at 0 ppm. DNP enhancement factors (ε) for both experiments were measured by running acquisitions with the microwaves switched on and off and comparing the resulting signal intensities.…”