Pseudocontact shifts in lanthanide complexes with variable crystal field parameters

Accurate pseudocontact shifts are the basis for structural determination in solution by means of paramagnetic NMR. Separation of pseudocontact (PCS) from Fermi contact (FC) shifts from NMR data can be achieved by means of the so-called Reilley method, which is briefly critically reviewed. It encounters a relevant limitation in the case of change of crystal field parameters through the series, as determined by various processes, primarily axial ligand dynamics, or as a consequence of lanthanide contraction. We propose a simple alternative procedure to compensate for any variation (smooth or abrupt) in crystal field parameters. Four examples taken from the literature, plus the complete set of unpublished data for Ln DOTMA are discussed in detail to illustrate the power and limitation of the conventional Reilley treatment and to demonstrate the power and scope of our alternative approach.