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.