Hyperpolarization of inorganic solids

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 (⍺-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.