7. Comments

Lanthanide shift reagents act like an additional magnetic field in the sample and dramatically change the chemical shifts of the signals, especially for nuclei in the vicinity of the complexation site ("the poor man's 1 GHz NMR spectrometer"). Thus, they make it possible to separate signals or to simplify an NMR spin system. The use of lanthanide shift reagents usually gives an unambiguous answer to problems such as cis/trans, E/Z, endo/exo or syn/anti assignments. The change in chemical shift resulting from the addition of a shift reagent can be expressed as the sum of three components, as given by Equation (1).

Δ = Δ_DIA + Δ_CON + Δ_DIP(1)

The diamagnetic contribution Δ_DIA is caused by the complexation of the substrate, whereas the contact contribution Δ_CON has its origin in the delocalization of electron spin density from the lanthanide ion to the substrate. Usually both are small and can be neglected. The dipolar contribution Δ_DIP depends on the distance r between the lanthanide ion and the nuclear spin being observed, and the angle θ between the principal magnetic dipolar axis of the complex and the distance vector. The complex usually has axial symmetry in solution and the McConnell–Robertson Equation (2) holds:

Δ_DIP = K (3cos²θ − 1) r⁻³(2)

This equation can be used to give quantitative information about the structure or conformation of a compound, if the bound shifts are known [3]. Lanthanide shift experiments are only successful when the substrate acts as a Lewis base to which complexing can occur. The degree of Lewis basicity decreases in the following order:

RNH₂ > ROH > RCOR > RCOOR > RCN

The commercially available lanthanide shift reagents are 1,3 diketone complexes. In contrast to europium complexes, those with praseodymium shift the signals to lower frequency, while the ytterbium reagents are best used in ¹³C NMR spectroscopy. Binuclear shift reagents such as Ag(fod)/Eu(fod)₃ can be used for unsaturated hydrocarbons.

8. Own Observations