Advances in the characterization of inorganic solids using NMR correlation experiments

Overview & Synopsis

As a local characterization technique endowed with atomic resolution, solid-state NMR spectroscopy provides unique insights on the atomic-level structure and dynamics of inorganic and hybrid materials. In particular, two-dimensional through-bond and through-space correlation experiments allow the observation of covalent bonds and proximities between identical or distinct isotopes, thus providing detailed information on the arrangement of atoms in the materials. Compared to biological and organic samples, inorganic and hybrid materials contain additional NMR-active nuclei, and notably quadrupolar isotopes with nuclear spin I ≥ 1, such as 11 B, 27 Al and 71 Ga, which are often subject to large quadrupolar interaction. Therefore, specific NMR correlation experiments have been developed to probe the local environment of these quadrupolar isotopes. We provide here an in-depth overview of correlation experiments, which have been employed for the characterization of inorganic and hybrid materials. We present first the through-space and through-bond correlation experiments to probe connectivities and proximities between identical nuclei, and then their counterparts for distinct isotopes. In both cases, we describe the experiments employed for spin-1/2 and quadrupolar isotopes. We indicate the state-of-the-art technique and the isotopes, for which they have been applied. Finally we present how these correlation NMR experiments have provided essential information on the atomic-level structure of different classes of inorganic hybrid materials, including (i) microporous materials (aluminophosphates, zeolites, metal-organic frameworks (MOFs)), (ii) metal oxide catalysts, such as amorphous silica alumina as well as supported metal complexes on alumina or silica, (iii) minerals and biomaterials and (iv) glasses.