Structure-independent method for dissecting contact and dipolar NMR shifts in lanthanide complexes and its use in structure determination

Abstract

Bleaney's theory of dipolar shifts or "contact free" proton shifts and Golding's theory of contact shifts provide, In joint consideration, bases for separation of the contribution each makes to the experimentally observed NMR shift. This treatment makes use of shifts obtained with several selected lanthanides but does not require a prior knowledge of the structure of the ligand adduct other than effective axial symmetry around the lanthanide. The resulting G factors for the various observed nuclei (1H, 13C, etc.) may be employed to orient a known fragment of the adduct molecule with answers to subsequent structural questions being addressed from additional observed shifts for nuclei and from constraints In their relative geometric placement. The procedure is applied to two organic compounds.

Conformational and Coordination Equilibria on DOTA Complexes of Lanthanide Metal Ions in Aqueous Solution Studied by 1H-NMR Spectroscopy

“…This is supported by our previous observation 22 that the contact/dipolar shift separation plots 30 obtained for the proton and 13 C resonances of the major and minor isomers do not show any large deviations from linearity, although the minor isomer changes from m to m′ along the lanthanide series.…”

Lanthanide Chelates Containing Pyridine Units with Potential Application as Contrast Agents in Magnetic Resonance Imaging

“…Assuming that hS z i j and C j values are the same for the complexes and the free ions, [25] for which they are tabulated, [26,27] , plots of d para ij /hS z i j against C j /hS z i j and of d para ij /C j against hS z i j /C j should be linear if the complexes are isostructural and possess comparable crystal field parameters. Plots of the paramagnetic shifts available for the Ln III complexes of L (Ln = Ce, Pr, Nd, and Sm) follow a linear correlation according to Equations (4) and (5), indicating that the complexes are isostructural in D 2 O (Figure 2).…”

NMR conformational study of the lanthanide(III) complexes of DOTA in aqueous solution

“…Table 1 contains the LIS values (at different temperatures) relative to the 1H and 13C signals of the two isomeric species of the Ln(DOTA)-chelates. The contact and pseudo-contact contributions to the LIS values were separated according to the method of Reilley [17,18] and the geometrical factor (G) was determined for each of the complexes. The calculation of the parameter G, as well as of the Ln-H distances within the complex, were also performed by a fitting procedure of the experimental NMR results to an axial symmetry model of the chelates (based on published X-ray data [19]) (Tables 2 and 3).…”