Lanthanide shift reagents. II. Shift mechanisms

Abstract

The lanthanide-induced NMR shifts in the complexes of Pr(fod) 3 with the model compounds 1,2-dimethoxyethane, 4,5-dimethylveratrole, and 1-methoxy-2-octyloxyethane are analyzed as a function of temperature. It is shown that a T −1 dependence is dominant, at variance with Bleaney's theory, which predicts a T −2 dependence. This rules out the application of the temperature dependence to separate Fermi contact and pseudocontact contributions to the induced shifts. Moreover, other methods currently used to accomplish this separation are inadequate for the systems investigated by us. In order to achieve, nevertheless, a separation of FC and PC contributions the lanthanide dependence of the shifts induced by Ln(fod) 3 in the rigid molecule, DMV was studied extensively. The data could be analyzed utilizing the full equation for the induced shift. It was found that for most lanthanides the Fermi contact interaction contributes considerably to the 1H and 13C shifts. The susceptibility tensors of the complexes investigated turn out to be nonaxial. Finally a criterion for the occurrence of apparent axial symmetry is presented and illustrated with some examples.