Modeling 129Xe NMR chemical shift sensitivity in carbon nanotube systems

Abstract

Understanding xenon nuclear magnetic resonance (NMR) in confined environments is essential for characterizing porous materials at the nanoscale. Here, we investigate how temperature and density of the xenon gas affect the 129Xe NMR chemical shift in pristine carbon nanotubes (CNTs) and CNT bundles using a multiscale computational approach. Canonical Monte Carlo (MC) simulations on static potential energy (PES) and chemical shift surfaces (CSS) are applied to capture the statistical behavior of Xe atoms, while molecular dynamics/DFT calculations provide insight into the role of CNT dynamics. At low Xe densities, the chemical shift is 5–13 ppm higher than at the static equilibrium position. Inside narrow CNTs, the chemical shift increases nearly linearly with temperature, whereas inside wider tubes and on the outer surface of a CNT, it exhibits nonlinear and decreasing trends at high temperatures. Interactions with other xenon atoms induce a strong positive density dependence of Xe shift that modifies the temperature behavior, producing a systematic increase in the chemical shift but also suppressing the simple linear temperature dependence observed at low densities inside CNTs. Molecular dynamics simulation reveals that inclusion of CNT motion increases chemical shifts inside a nanotube by 20–30 ppm. At high densities, the adsorption on the groove site results in a rise of chemical shift distribution around the 200 ppm range. These simulations provide microscopic insight into how structural, thermal, and interaction-driven effects govern xenon NMR responses in carbon nanotube environments, and therefore provide guidance for interpreting experimental observations across a range of temperatures and densities.