Magnetic anisotropy of endohedral lanthanide ions: paramagnetic NMR study of MSc2N@C-80-I-h with M running through the whole 4f row

摘要

Paramagnetic and variable temperature C-13 and Sc-45 nuclear magnetic resonance studies are performed for nitride clusterfullerenes MSc2N@C-80 with icosahedral Ih(7) carbon cage, where M runs through all lanthanides forming nitride clusters. The influence of the endohedral lanthanide ions on the NMR spectral pattern is carefully followed, and dramatic differences are found in peak positions and line widths. Thus, C-13 lines broaden from 0.01-0.02 ppm in diamagnetic MSc2N@C-80 molecules (M = La, Y, Lu) to several ppm in TbSc2N@C-80 and DySc2N@C-80. Direction of the paramagnetic shift depends on the shape of the 4f electron density in corresponding lanthanide ions. In TmSc2N@C-80 and ErSc2N@C-80 with prolate 4f-density of lanthanide ions, C-13 signals are shifted down-field, whereas Sc-45 peaks are shifted up-field versus diamagnetic values. In all other MSc2N@C-80 molecules lanthanide ions have oblate-shaped 4f electron density, and the lanthanide-induced shift is negative for C-13 and positive for Sc-45 peaks. Analysis of the pseudocontact and contact contributions to chemical shifts revealed that the pseudocontact term dominates both in C-13 and Sc-45 NMR spectra, although contact shifts for C-13 signals are also considerable. Point charge computations of the ligand field splitting are performed to explain experimental results, and showed reasonable agreement with experimental pseudocontact shifts. Nitrogen atom bearing large negative charge and located close to the lanthanide ion results in large magnetic anisotropy of lanthanide ions in nitride clusterfullerenes with quasi-uniaxial ligand field.