Mapping Out Chemically Similar, Crystallographically Nonequivalent Hydrogen Sites in Metal–Organic Frameworks by 1H Solid-State NMR Spectroscopy

Metal–organic frameworks (MOFs) are important materials with many actual and potential applications. Crystal structure of many MOFs is determined by single-crystal X-ray diffraction. However, due to the inability of XRD to accurately locate hydrogen atoms, the local structures around framework hydrogen are usually poorly characterized even if the overall framework has been accurately determined. 1H solid-state NMR (SSNMR) spectroscopy should, in principle, be used as a complementary method to XRD for characterizing hydrogen local environments. However, the spectral resolution of 1H SSNMR is severely limited by the strong 1H–1H homonuclear dipolar coupling. In this work, we demonstrate that high-resolution 1H MAS spectra of MOF-based material can be obtained by ultrafast sample spinning at high magnetic field in combination with isotopic dilution. In particular, we examined an important MOF, microporous α-Mg3(HCOO)6 and α-Mg3(HCOO)6 in the presence of several guest species. All six chemically very similar,...