Analysis of lanthanide induced NMR shifts using a crystal field independent method: Application to complexes of tetraazamacrocyclic ligands

The linear graphical analysis of the LIS NMR data available for the axially symmetric complexes [Ln(DOTA)]^- (M and m isomers), [Ln(DOTP)]^5- (pH 10.0, 7.0 and 3) and [Ln(DOTEA)]³⁺ using the classical crystal field dependent method and a crystal field independent method were compared. As the second method provides ratios of geometric structural terms G rather than G values, the effect of lanthanide contraction was reduced. Thus, the large breaks in plots observed for all nuclei of those systems using the classical method are still present in the plots of the second method, only in a few of the nuclei and much reduced. This shows that the large breaks at the middle of the lanthanide series present in plots of the classical method as well as the anomalies often present for those plots for the Tm and Yb ions are mostly due to changes of the crystal field coefficient A₂⁰〈r²〉 along the lanthanide series, while both the hyperfine coupling constants and the ratios of geometric terms also change as a result of the lanthanide contraction, leading to small breaks at the middle of the lanthanide series. Analysis of the proton shifts of [Ln(DOTP)] complexes at pH 10, 7 and 3 indicates that protonation of the complexes results in a decrease on the crystal field coefficient. The dipolar shift ratios and absolute shifts obtained were also interpreted in terms of the structural models for those complexes in solution and their available X-ray crystal structures.