Lanthanide-induced shifts pseudocontact shift

Lanthanide Shift Reagents.—The effects of random co-ordinate error in analysis of lanthanide-induced axial pseudocontact shifts have been discussed, " and the contributions of contact and pseudocontact shifts in the n.m.r. spectra of isoquinoline and of endo-norbornenol have been evaluated. An experimental and computational approach to the use of lanthanide-induced shifts as a rigorous method for structure determination has been described. The method was used to predict the lanthanide-induced shift behaviour of a substrate. The recording of experimental data in excellent agreement with the molecular structure was reported. Contact shift contributions to lanthanide isotropic shifts have been found to be important for organic compounds even where the carbon atom is five bonds away from the lanthanide. 

The paramagnetic properties of the lanthanides have been exploited in NMR spectroscopy for many years [16,17], The paramagnetic lanthanide induced shift is generally considered as the sum of the contact and pseudocontact terms (Equation 1) [16-18]... 

The lanthanide induced shifts (LIS) arise from the interaction of the magnetic nucleus with the magnetic field of the paramagnetic lanthanide ion. This could arise via two mechanisms either a pseudocontact (dipolar) through-space fashion involving a dipole dipole interaction, or by a contact mechanism with delocahzation of unpaired spin density from the lanthanide f-orbitals via the lanthanide substrate bond on to the organic molecule the former is more important. 

Quantitative separation of the n contact and pseudocontact contributions to the lanthanide induced shifts (LIS) in aniline and m-and p-toluidines has been reported. (395, 396) The contact shift patterns are estimated from the rr-spin density distribution of the appropriate cation radical or from the Ni(acac)2 induced shifts. The separation of the shifts was checked by comparing the relative contact shift contribution with the <5 ) value of Golding and Halton (389) and the remaining pseudocontact contribution with the calculated values of Bleaney s theory. (380)... 

Chemists did not follow the caution voiced by Weissman [30] that both contact and pseudocontact (dipolar) shifts are to be expected in molecules coordinated to Europium shift reagents. Numerous workers applied a simplified form of the McConnell Robertston relationship (Equation (3)) for pseudocontact interaction as the sole interpreter of the lanthanide induced shifts in rigid organic molecules. It has been shown that especially when C data are compared to H data [31], that both the dipolar and contact effects operate simultaneously ... 

Ln3+ induced water 170 shifts of [Ln(DOTA)] solutions show that the hydration number of the complexes is one across the lanthanide series [59]. The substantial pseudocontact contribution to its LIS indicated that this water ligand has a preferred location in the complex. Two sets of peaks have been observed in H and 13C NMR spectra of [Ln(DOTA)] complexes at room temperature showing the presence of two slowly interconverting structural isomers [60-63]. In the spectra of the paramagnetic complexes, one isomer has larger LIS values than the other. These structural features have been confirmed by luminescence studies [51, 64]. The temperature dependence of the H and 13C NMR spectral features of both the dia- and paramagnetic Ln3+ complexes indicates that the... 

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 ... 

Only the pseudocontact shifts give exact information on the location of the cation, and, therefore, on the structure of the complex. According to an equation developed by McConnell and Robertson,the lanthanide-induced shift, Av, is... 

Lanthanide ions with anisotropic molecular magnetic susceptibility tensor Ay, for example, Er , Tm , or Yb , allow valuable structural information to be extracted from pseudocontact shifts (PCS, see also Section 3.6) and residual dipolar couplings (RDCs) arising from the partial alignment of the -labeled biomolecules in the induced magnetic field (Su et al., 2008b) ... 

The binding of a ligand to a paramagnetic lanthanide ion induces frequency shifts in the NMR spectrum (LIS) of the ligand relative to that observed for the corresponding diamagnetic complex. The LIS has two contributions, the Fermi contact (6c) and the dipolar or pseudocontact shifts (5pc) ... 

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