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Solid-state NMR of low-gamma nuclei and solid-state NMR probe design towards minimization of the perturbation of the applied magnetic field
Schönzart, Jasmin
Schönzart, Jasmin
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2024
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2025-11-26
Abstract
While many techniques can be used for structural analysis, none of them are able to probe chemical bonding and electronic structurewith the level of detail that can be obtained through non-destructive solid- state NMR spectroscopy (SSNMR). The SSNMR approach can be used to obtain details about molecular- level structure andmetal-ligand interactions, leading to deeper understanding of structure-function-property relationships. In addition to experimental techniques, the design of the instrumentation for SSNMR is very challenging to obtain sufficient resolution, varietyof temperatures and sample spinning ranges. In this thesis, my recent work in three different areas will be presented: the first will illustrate howwe have expanded the ability to conduct NMR experiments for unreceptive nuclei (i.e. nuclei with a low gyromagnetic ratio (γ)); and second illustrate howwe have expanded the application ofMagic Angle spinning (MAS) NMR through the development ofunique probemodules to be able to resolveNMRresonances in the part per billion (ppb) magnitude in the solid-state. Those learned techniques are then used to build not only the probe, but also so-called pocket magnets with a field homogeneity sufficient for NMR at ultra-high field strengths. The low-γ nuclei study demonstrates the application of 103Rh solid-state NMR (SSNMR) spectroscopy to inorganic and organometallic coordination compounds, in combinationwith relativistic density functional theory (DFT) calculations of 103Rh chemical shift tensors to develop correlations between 103Rh chemical shift tensors, molecular structure, andRh-ligand bonding. 103Rh is one of the least receptiveNMRnuclides, and consequently, there are very few reports in the literature of solid-state NMR spectra. Robust 103Rh SSNMR protocols for stationary samples, which use the broadband adiabatic inversion-cross polarization (BRAIN-CP) pulse sequence andwidebanduniform-rate smooth-truncation (WURST) pulses for excitation, refocusing, and polarization transfer are presented, which demonstrate the acquisition of 103Rh SSNMR spectra of unprecedented signal-to-noise and uniformity. Magic-angle spinning (MAS) solid-stateNMRmethods are crucial inmanyareas ofbiologyandmaterials science. Conventional probe designs have often been specified with 0.1 part per million (ppm) or 100 part per billion (ppb) magnetic field resolution, which is a limitation for many modern scientific applications. Here, a novel 5-mmMAS module design that significantly improves the linewidth and line shape for solid samples is described. This is achieved by an improved understanding of themagnetic susceptibility of probe materials and geometrical symmetry considerations, optimized to minimize the overall perturbation to the applied magnetic field (B0). The improved spinning module requires only first and second order shimming adjustments to achievea sub-Hertz resolutionof13C resonances ofadamantaneat 150MHz Larmor frequency (14.1 Tesla magnetic field). iiiproposed to buildmagnets above 50 Tesla for wide dissemination to the NMRcommunity. High temperature superconductor (HTS) with high current densities will readily provide a viable path to a newgeneration ofmagnets for NMRwhich are smaller, better, and cheaper. Conventional NMRprobes are far too large in diameter to fit into such magnet bores. Instead, NMR probes need to be narrow (<10 mm diameter), short (<20 mm) and designed considering magnetic susceptibilityof components near the sample to ultimately acquire high resolution NMR spectra.
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