Acquiring Focus on Paramagnetic Single-Atom Sites with Fast Magic-Angle Spinning NMR

A new approach for characterizing paramagnetic sites in materials is introduced. It combines broadband fast magic-angle spinning (MAS) NMR data with ab initio computed paramagnetic NMR shifts using correlated wave functions. This study presents a challenging example of this. With Fe coordinated in a model compound, the PCN-224 porphyrin metal-organic framework (Fe@PCN-224 MOF) was used to elucidate the coordination geometry and electronic structure using 1H and 13C MAS NMR spectra of the ligand atoms. The computationally predicted 13C NMR shifts on the paramagnetic Fe@PCN-224 MOF compared unprecedentedly well with experimental 13C NMR shifts and equally well for the diamagnetic counterpart, the Fe-free PCN-224 MOF. This is despite the 25 times wider NMR shift range of 1200 ppm for the paramagnetic Fe@PCN-224 MOF. We conclude that this approach is applicable to crystalline, noncrystalline, and molecular systems.