Maximizing nuclear hyperpolarization in pulse cooling under MAS

It has recently been shown how dynamic nuclear polarization can be used to hyperpolarize the bulk of proton-free solids. This is achieved by generating the polarization in a wetting phase, transferring it to nuclei near the surface and relaying it towards the bulk through homonuclear spin diffusion between weakly magnetic nuclei. Pulse cooling is a strategy to achieve this that uses a multiple contact crosspolarization sequence for bulk hyperpolarization. Here, we show how to maximize sensitivity using the pulse cooling method by experimentally optimizing pulse parameters and delays on a sample of powdered SnO2. To maximize sensitivity we introduce an approach where the magic angle spinning rate is modulated during the experiment: the CP contacts are carried out at a slow spin rate to benefit from faster spin diffusion, and the spin rate is then accelerated before detection to improve line narrowing. This method can improve the sensitivity of pulse cooling for Sn-119 spectra of SnO2 by an additional factor of 3.5. (C) 2019 Elsevier Inc. All rights reserved.

Dynamic Nuclear Polarization and Other Magnetic Ideas at EPFL

Geoffrey Bodenhausen, Aurélien Bornet, Roberto Buratto, Marc Anthony Caporini, Diego Carnevale, Srinivas Chinthalapalli, Sami Jannin, Daniele Mammoli, Pascal Miéville, Jonas Milani, Angel Joaquin Perez Linde, Martial Rey, Nicola Salvi, Simone Ulzega, Shutao Wang

Although nuclear magnetic resonance (NMR) can provide a wealth of information, it often suffers from a lack of sensitivity. Dynamic Nuclear Polarization (DNP) provides a way to increase the polarization and hence the signal intensities in NMR spectra by transferring the favourable electron spin polarization of paramagnetic centres to the surrounding nuclear spins through appropriate microwave irradiation. In our group at EPFL, two complementary DNP techniques are under investigation: the combination of DNP with magic angle spinning at temperatures near 100 K ('MAS-DNP'), and the combination of DNP at 1.2 K with rapid heating followed by the transfer of the sample to a high-resolution magnet ('dissolution DNP'). Recent applications of MAS-DNP to surfaces, as well as new developments of magnetization transfer of H-1 to C-13 at 1.2 K prior to dissolution will illustrate the work performed in our group. A second part of the paper will give an overview of some 'non-enhanced' activities of our laboratory in liquid- and solid-state NMR.

Schweizerische Chemische Gesellschaft2012

Hyperpolarization of inorganic solids

Snaedis Björgvinsdóttir

Solid-state NMR can provide information about the atomic level structure and dynamics of materials. It directly probes symmetry and structure at nuclear sites, and is especially useful for investigation of disordered or amorphous solids that lack long range order. However, the application of solid-state NMR is sometimes limited by its relatively low sensitivity, caused by low concentrations and low gyromagnetic ratios of the magnetically active nuclei. Dynamic nuclear polarization (DNP) can provide significant signal enhancements in magic-angle-spinning (MAS) NMR experiments. This is usually achieved by introducing stable organic radicals to the sample of interest, and transferring their large electron spin polarization to nearby nuclear spins via microwave irradiation near to the EPR frequency. Methods to hyperpolarize the bulk of solid materials containing protons are well established, whereas NMR of proton-free bulk materials remains challenging in many cases, especially when nuclear relaxation times are long, and if isotopic enrichment or paramagnetic doping to enhance relaxation rates are not feasible. The overall objective of the work described in this thesis is to improve sensitivity in DNP enhanced solid-state NMR experiments, and to extend the application of DNP to systems that are currently difficult to access. In particular, this includes developing a strategy to hyperpolarize the bulk of proton-free inorganic materials. In the first part, the classic flip-back method to recover bulk proton magnetization is combined with DNP of 1H containing solids with characteristic build-up times spanning two orders of magnitude. Gains in sensitivity in the 13C spectra of powdered crystalline theophylline, histidine and salicylic acid are reported, on top of the enhancements already provided by relayed DNP. In the second part, a general strategy where the relayed DNP method is extended to proton-free inorganic materials is reported. The method uses a combination of impregnation DNP and slow spin diffusion between weakly magnetic nuclei such as 119Sn and 31P. Hyperpolarization is continuously generated at the surface either by direct DNP of the weakly magnetic nuclei, or by multiple bursts of cross polarization (CP) from protons in the wetting phase. Provided that bulk T1 values are long, even slow spin diffusion can then transfer the surface-generated hyperpolarization to the bulk, resulting in spectra which exceed the sensitivity of conventional solid-state NMR. As an example, multiple contact CP can provide a factor 50 gain in overall sensitivity of the 119Sn spectrum of SnO2, allowing access to materials that were previously unfeasible to study. Overall in this thesis, hyperpolarization transport by spin diffusion is confirmed between 31P nuclei in GaP and Sn2P2O7, 119Sn in SnO2, 113Cd in CdTe, 29Si in SiO2 (âº-quartz) and 6Li/7Li in lithium titanates, and shown to improve sensitivity in their bulk NMR spectra. Strategies to optimize bulk hyperpolarization are shown, as well as two-dimensional spin diffusion experiments which provide insight into the process of polarization transfer from surface to bulk. In the third part, DNP is explored at the highest field and spinning frequencies available for DNP to date. NMR signal enhancements of 200 are reported at 21.1 T, enabled by 65 kHz MAS at 100 K. The fast spinning frequencies also yield high resolution DNP enhanced 1H-detected heteronuclear correlation spectra.

EPFL2021

Hyperpolarization of inorganic solids

Snaedis Björgvinsdóttir

EPFL2021