A recent scientific seminar showcased methodological advances in solid-state nuclear magnetic resonance (NMR) spectroscopy, with presentations addressing the characterization of paramagnetic energy-storage materials, the development of broadband recoupling pulse sequences, and recent progress in dynamic nuclear polarization (DNP)-enhanced biomolecular NMR. Together, the studies illustrated how advances in computational modelling, pulse sequence design, and experimental optimization continue to expand the capabilities of modern solid-state NMR for structurally complex systems.
The first presentation focused on Prussian Blue Analogues (PBAs), a class of open-framework coordination compounds widely investigated as cathode materials for sodium-ion batteries. Despite their well-defined average crystallographic structures, accurately determining the local distribution of sodium ions and water molecules remains challenging because diffraction-based refinements often produce multiple structural models with comparable agreement factors.
Solid-state ^23Na NMR spectroscopy was employed to investigate the local sodium environments in manganese-containing PBAs. Computational analyses were performed to evaluate the relative contributions of pseudo-contact interactions, bulk magnetic susceptibility, and hyperfine interactions to the experimentally observed paramagnetic chemical shifts. The results indicated that direct orbital overlap between manganese and sodium electronic states is the dominant mechanism responsible for the large ^23Na chemical shifts.
To further investigate sodium distribution within the framework, computational modelling based on cluster-expansion approaches and machine learning was combined with neutron diffraction and pair distribution function analyses. The integrated study indicated preferential sodium localization in the vicinity of manganese coordination environments, while comparison with nickel-based analogues suggested greater sodium disorder and mobility, consistent with their improved electrochemical rate performance.
The second presentation described the implementation of adiabatic inversion pulses in Radio Frequency Driven Recoupling (RFDR) experiments to improve polarization transfer in systems exhibiting broad resonance offsets and large chemical shift anisotropies.
Numerical simulations demonstrated substantially improved transfer efficiency over conventional rectangular π pulses across broad frequency ranges. Experimental validation on pharmaceutical, inorganic, and biomolecular samples confirmed enhanced signal intensity, improved spectral resolution, and greater robustness against resonance offset while preserving structural information derived from dipolar couplings.
The methodology was further extended to proton-detected experiments under fast magic-angle spinning (MAS), enabling improved observation of long-range proton correlations in systems characterized by extensive spectral dispersion.
The final presentation addressed current developments in DNP-enhanced MAS NMR spectroscopy for biomolecular applications. While DNP provides significant gains in sensitivity, the low temperatures required for polarization transfer frequently reduce spectral resolution owing to increased line broadening and the presence of paramagnetic radicals.
Strategies for mitigating these effects included optimization of sample formulation, implementation of proton-detected experiments, extensive deuteration, selective methyl protonation, and fast MAS. Comparative evaluation of soaking and co-crystallization approaches demonstrated a balance between spectral resolution and polarization enhancement, depending on the experimental objective.
Application of these methodologies to protein systems demonstrated substantial sensitivity enhancement while maintaining sufficient spectral resolution for resonance assignment and extraction of structurally relevant proton–proton distance restraints.
The seminar concluded with a discussion of the experimental challenges associated with acquiring ^23Na NMR spectra from paramagnetic sodium battery materials, particularly under conditions of large chemical shift anisotropy and extensive spinning sideband manifolds.
To improve spectral interpretation, participants proposed implementing Cross Polarization Magic Angle Turning (CPMAT) or related angle-turning experiments. These experiments enable determination of chemical shift anisotropy parameters and facilitate discrimination between isotropic resonances and spinning sidebands, thereby improving characterization of sodium coordination environments in paramagnetic materials.
The planned experimental work will evaluate these methodologies on sodium battery samples with the objective of establishing optimized protocols for the investigation of broad ^23Na spectra in paramagnetic electrode materials.
The seminar highlighted ongoing methodological developments that integrate advanced computational approaches with innovations in pulse sequence design and DNP methodology. Collectively, these advances continue to strengthen the role of solid-state NMR as a powerful tool for investigating the local structure and dynamics of functional materials and biomolecular assemblies.