Noy Vaisleib | Searcharxiv

arXiv2025

Enhanced 133cs Triple-Quantum Excitation in Solid-State NMR of Cs-Bearing Zeolites

Geopolymers are aluminosilicate materials that exhibit effective immobilization properties for low-level radioactive nuclear waste, and more specifically for the immobilization of radioactive cesium. The identification of the cesium-binding sites and their distribution between the different phases making up the geopolymeric matrix can be obtained using solid-state NMR measurements of the quadrupolar spin 133Cs, which is a surrogate for the radioactive cesium species present in nuclear waste streams. For quadrupolar nuclei, acquiring two-dimensional multiple-quantum experiments allows the acquisition of more dispersed spectra when multiple sites overlap. However, 133Cs has a spin-7/2 and one of the smallest quadrupole moments, making multiple-quantum excitation highly challenging. In this work we present pulse schemes that enhance the excitation efficiency of 133Cs triple quantum coherences by a factor of ~2 with respect to a two-pulse excitation scheme. The improved schemes were developed by using numerical simulation and verified experimentally by applying one and two-dimensional triple-quantum solid-state NMR experiments to a mixture of cesium-exchanged hydrated zeolites A and X, which possess dynamically averaged small quadrupolar coupling constants in the order of 10 kHz.

physics.chem-ph↗

arXiv2025

Prediction of NMR Parameters and geometry in 133Cs-containing compounds using Density Functional Theory

The need to immobilize low-level nuclear waste, in particular 137Cs-bearing waste, has led to a growing interest in geopolymer-based waste matrices, in addition to optimization attempts of cement matrix compositions for this specific application. Although the overall phase composition and structure of these matrices are well characterized, the binding sites of Cs in these materials have not been clearly identified. Recent studies have suggested that combining the sensitivity of solid-state Nuclear Magnetic Resonance (SSNMR) to the local atomic structure with other structural techniques provides insights into the mode of Cs binding and release. Density Functional Theory (DFT) can provide the connection between spectroscopic parameters and geometric properties. However, the reliability of DFT results strongly relies on the choice of a suitable exchange-correlation functional, which for 133Cs, the NMR surrogate for such studies, is not well-established. In this work we benchmark various functionals against their performance in predicting the geometry of various simple Cs compounds, their NMR quadrupolar coupling constants, and their chemical shift values, while prioritizing the ability to incorporate dispersion interactions and maintaining low computational cost. We examined Cs salts, Cs oxides, perovskites, caged materials, a borate glass and a cesium fluoroscandate. While no single functional performs equally well for all parameters, the results show rev-vdW-DF2 and PBEsol+D3 to be leading candidates for these systems, in particular with respect to geometry and chemical shifts, which are of high importance for Cs-immobilization matrices.

physics.chem-ph↗