Proton chemical shifts lanthanide complexes

Information on the hydration state of the Gd(III) chelate in solution is indispensable for the analysis of its proton relaxivity Several methods exist to determine q, though they are mostly applicable for other lanthanides than Gd(III). In the case of Eu(III) and Tb(III) complexes, the difference of the luminescence lifetimes measured in D20 and H20 can be related to the hydration number [15, 16]. For Dy(III) chelates, the lanthanide induced 170 chemical shift of the bulk water is proportional to the hydration number [17]. Different hydration states of the same chelate may also coexist in solution giving rise to a hydration equilibrium. Such an equilibrium can be assessed by UV-Vis measurements on the Eu(III) complex [18-20]. These techniques have been recently discussed [21]. 

Fig. 2. A plot of the chemical shifts of the proton signals (ppm) for the 1,10-phenanthroline complexes of the lanthanides in D2O. The shifts are below the methyl signals of tert-butyl alcohol...

After protons, C is the most widely detected nucleus in NMR. Proton cross-polarization and decoupling are usually applied to increase the S/N, and these types of experiment can result in substantial sample heating. Many forms of C-based NMR thermometers have been proposed. The first such system was based on the cis-trans conformational equilibrium of furfural, with the linewidths of carbon-3 and the aldehyde carbon being temperature-dependent. There are many disadvantages of linewidth-based measurements, and subsequent developments concentrated almost wholely on temperature-dependent C chemical shifts. The first such system utilized a temperature-dependent lanthanide-induced pseudocontact shift in a complex of acetone-de and ytterbium(III)1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octadionate (Yb (fod)3). The 6co of the acetone-dg, measured with respect to a CS2 standard, was almost linearly dependent on 1 / T with a small quadratic term over a range 200-315 K. If a small amount of protonated acetone was added, then the proton resonance, measured with respect to the protons of TMS, was also found to be temperature dependent ... 

The equilibrium constant for the complexation of cholesterol with Eu(fod)3 has been evaluated from measurements on a series of solutions at varying total concentrations but identical molar ratio. The same analysis provides values for the chemical shifts of protons in the uncomplexed steroid, and in the steroid-europium complex the latter value is not accessible by direct measurement, since the complex is always in equilibrium with the free steroid. The mathematical equations presented in this paper should be generally applicable. In another paper the validity of various procedures for interpreting lanthanide-induced shifts is explored comments on cholesterol are included. No one mathematical model at present available is considered to have general validity. 

Nuclear Magnetic Resonance Spectroscopy.—As noted above, conformational analysis of bicyclo[3.3.1]nonanes is still a topic of considerable interest. A variable-temperature n.m.r. analysis now provides the first case in which the boat-chair-chair-boat equilibrium is directly observed in the amines (17) and (18). In a related case, re-examination of the acetal (19) suggests that the preferred conformation involves a chair carbocyclic ring and a boat heterocyclic ring. This conclusion was made by n.m.r. analysis, using lanthanide shift reagents, by a study of nuclear Overhauser effects, and by measurement of relaxation times of protons. Details have been reported for other 3-azabicyclo[3.3.1]nonanes, and the non-additivity of substituent effects on chemical shifts in 9-thiabicyclo[3.3.1]non-2-enes has been analysed. Both and n.m.r. data have been reported for a series of 9-borabicyclo[3.3.1]non-anes and their pyridine complexes. 

Solution structures deduced from LIS data. - Andre et a/." have prepared new ditopic trihelicate lanthanide (La-La -L3) complexes, which they consider of potential utility as biomedical probes. The solution structures of these species were determined by analyses of lanthanide-induced chemical shifts and proton spin relaxation data of mixed complexes containing one diamagnetic Lu + ion and one paramagnetic lanthanide ion. 

Valencia et have used proton pseudocontact chemical shift data in conjuction with x-ray crystal structures to determine the solution geometries of the lanthanide complexes with the ligand Py2N6Ac4, which contains four acetate pendant arms. The aqueous solution structures of diamagneticLa and Lu complexes were characterized by their COSY NMR spectra. The structures of the paramagnetic complexes were determined by fits of H pseudocontact chemical shift data and spin relaxation data to a model that assumed rhombic magnetic susceptibility tensors. 

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