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

TL;DR

This study benchmarks density functional theory functionals to accurately predict NMR parameters and geometry of Cs compounds, aiding the understanding of Cs binding in waste matrices.

Contribution

It identifies suitable DFT functionals for modeling 133Cs NMR parameters and geometry in various compounds relevant to nuclear waste immobilization.

Findings

rev-vdW-DF2 and PBEsol+D3 perform best for geometry and chemical shifts

No single functional excels across all parameters

Benchmarking guides functional choice for Cs-containing materials

Abstract

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.