Paramagnetic lanthanide shift reagents in NMR spectroscopy: Principles, methodology and applications

The lanthanide ions as structural probes in biological and model systems

“…The contact mechanism is thus of least importance for protons. Reuben and Fiat (10,76,122) have confirmed, albeit tentatively, that for the protons of RE bound water mocules the contact interaction was secondary importance. On the basis of this work and other "oversimplified" calculations (JO), the greatest interaction is predicted for Eu+~.…”

“…This conclusion is consistent with the accepted notion that 8/orbitals are too well shielded to participate in direct orbital overlap with ligand molecular orbitals. For the lanthanides, the fundamental relationship describing the contact shift (10,120,121) is given in Eq. (8).…”

“…Large contact shifts have been assigned to all proton shifts of substituted pyridines induced by interaction with Pr +a and Nd +3 salts in acetonitrile (18,123). However, the data analysis for this system has recently been criticized (10).…”

“…Even though the full form of Eq. Reuben (10), and should be applicable to studies in aqueous media. A novel and versatile approach to such equilibrium studies will be mentioned in a subsequent section.…”

“…For the other RE ions the process is complicated by contributions due to anisotropic g-tensors (10,133,134). The relaxation rates for protons of a metal-bound ligand molecule are summarized in Eq.…”