A Quantitative Study of Effects of Rare Earth Chelates on NMR Chemical Shifts: I. Treatment of 1:1 Complexes and Chemical Shifts of the Pure Complexes

Recently several rare earth chelates have been found that induce large and dramatic chemical shifts when complexed with a ligand.¹ This is due to paramagnetic shielding, the mechanism of which is observed to be a pseudo-contact shift. In cases such as this involving dynamic equilibria where exchange of protons is rapid compared to observation time, the observed chemical shift represents an average environment determined by the time the proton nuclei spend at each site. The effective site of the complex is a lone pair donor such as the oxygen of a carbonyl group, alcohol, or ether, or the nitrogen of various nitrogen containing groups. The oxygen bearing compounds could theoretically form strong complexes with the added possibility of multiple complexes with chelates. The nitrogen bearing compounds of weak donor strength and containing only a lone pair would presumably form only the 1:1 complex.