Hyperpolarization transfer pathways in inorganic materials

Dynamic nuclear polarization can be used to hyperpolarize the bulk of proton-free inorganic materials in magic angle spinning NMR experiments. The hyperpolarization is generated on the surface of the material with incipient wetness impregnation and from there it is propagated towards the bulk through homonuclear spin diffusion between weakly magnetic nuclei. This method can provide significant gains in sensitivity for MAS NMR spectra of bulk inorganic compounds, but the pathways of the magnetization transfer into the material have not previously been elucidated. Here we show how two-dimensional experiments can be used to study spin diffusion from the surface of a material towards the bulk. We find that hyperpolarization can be efficiently relayed from surface sites to multiple bulk sites simultaneously, and that the bulk sites also engage in rapid polarization exchange between themselves. We also show evidence that the surface peaks can exchange polarization between different sites in cases of disorder. (C) 2020 The Author(s). Published by Elsevier Inc.

Automated summary

Dynamic nuclear polarization is used to hyperpolarize proton-free inorganic materials in magic angle spinning NMR experiments, with hyperpolarization generated on the surface and propagated towards the bulk through spin diffusion. Two-dimensional experiments are used to study spin diffusion from the surface to the bulk, showing efficient transfer of hyperpolarization from surface to multiple bulk sites simultaneously, as well as rapid polarization exchange between bulk sites. Evidence is also presented for polarization exchange between different sites on the surface in cases of disorder.