Comparison of different methods for structural analysis of lanthanide-induced NMR shifts: a case of lanthanide(III) cryptates

Abstract

The three main methods available in the literature for lanthanide-induced shift (LIS) structural analysis of a series of lanthanide(III) complexes in solution are applied to the data available for a complete series of paramagnetic lanthanide cryptate complexes of a Schiff base axial macrobicyclic ligand L, [LnL]3+. The macrobicycle contracts its cavity as a result of the lanthanide contraction, while preserving the same overall structure in solution. This causes breaks in the LIS plots for all the three methods used. A combined analysis of the data allows to conclude that those breaks reflect abrupt changes of the crystal field parameter A20<r2> and the hyperfine constants Fi of the lanthanides in the middle of the series, but not of the structure of the complex in solution.