Lanthanides contact shift

Fig. 2.21. Calculated patterns of dipolar shifts ( ) [79] and —(5.) ( ) [81] values (proportional to the contact shifts) induced by lanthanide ions.

Separation of pseudocontact and contact shifts in axial systems can also be achieved, again by using a variety of lanthanides, without assuming that for a given signal the contact shift is zero, as long as the expected patterns of both pseudocontact and contact shifts (Fig. 2.21) hold. Here, the hyperfine shift can be expressed as the sum of contact and pseudocontact shifts (Eqs. (2.39) and (2.37)) ... 

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]... 

Contact shifts and through-bond spin de-localisation in paramagnetic lanthanide cally rigid ligands 3.1. Complexes with threefold symmetry ... 

S 3 paramagnetic hyperfine chemical shift in ppm for a nucleus i in a complex of a lanthanide j paramagnetic contact shift in ppm for a nucleus i in a complex of a lanthanide j <5 ° paramagnetic pseudo-contact shift in ppm for a... 

Contact shifts and through-bond spin delocalisation in paramagnetic lanthanide complexes... 

Since this dipolar interaction remains in isotropic media as similarly found for the contact shift (see sect. 2.1), it is often termed as the pseudo-contact shift. For the general case of a complex possessing an anisotropic magnetic susceptibility tensor, Kemple et al. (1988) show that the pseudo-contact shift spherical coordinates of the resonating nucleus in an arbitrary axes system with the lanthanide metal ion i (III) located at the origin (fig. 1)... 

We will now proceed to a discussion of LIS methodologies, uses of lanthanides in complex NMR spectra, quantitative aspects of the LIS method such as testing and separation of shifts, use of pseudo contact shift in molecular structure, and use of aqueous cations and lanthanide complexes in biological systems. 

Relative dipolar shifts and their ratio to the contact shifts for lanthanide complexes (at 300 K). 

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. 

In addition to wrong-way shifts, there are other methods to assess whether contact shifts occur for a nucleus. One is to compute sets of internal ratios of the shifts of protons for different metals or for directly attached H or C nuclei. Anomalies in these ratios are indicative of a contact contribution. Chelates of gadolinium, which has an isotropic f configuration, can only produce shifts by complexation or contact effects . Theoretical parameters for the relative contributions of contact and dipolar shifts for different lanthanide metals can be used to construct plots that indicate whether or not the shifts of a particular nucleus are purely dipolar or have a contact contribution Theoretical parameters indicate that the proportion of contact contribution to the shifts occurs in the order Eu > Nd > Ho > Er > Tb > Dy > Tm > Using the theoretical values... 

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)... 

Lanthanide complexes [Ln(D02A)(H20) ]+ of the potentially hexadentate D02A have been studied by measuring the lanthanide-induced shifts in the O-NMR spectra. Analysis of the contact contribution indicates a decrease in the hydration number from = 3 (Ln = Ce-Eu) to n = 2 (Ln = Tb-Lu). Study of the Dq transition in the UV-visible spectra of the Eu complex... 

Lanthanide induced chemical shifts have been studied.280 l,2-Dithiole-3-thiones did not give any contact shifts with Eu(fod)3 and Pr(fod)3, whereas l,2-dithiol-3-ones gave moderate shifts. Attempts to define the position of lanthanide complexing were made for the l,2-dithiol-3-ones it was not possible to define the position of the lanthanide atom closer than 2-3 A from the oxygen and 40-50 on either side of the C—O line. 

Structure Probes.—The introduction of fluorescing labels into nucleic acids can yield valuable structural information, and both organic compounds and metal ions [Tb (ref. 136) and Eu (ref. 137)] have been used as fluorescent probes for tRNA and other polynucleotides. The degree of secondary structure in RNA has been estimated from Raman scattering by the phosphate group vibrations. A number of n.m.r. studies have appeared, but discussion of these is more suited to a review on n.m.r. spectroscopy. Lanthanide ions have been used as contact shift reagents to probe tRNA structure. ... 

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. 

Figure 8 shows that in Ln2Sn20y compounds (Ln = Eu, Tm, Yb, Sm, La, Pr, Nd), the experimental isotropic chemical shifts for the " Sn resonance correlate nicely with theoretical values calculated from Eq. (5) by assuming no variation of a with the rare earth cation considered. This result suggests that the Fermi contact shift contribution dominates in such cases. By contrast, the resonance shifts measured in lanthanide-substituted compounds Y2 -, LniSn207 and... 

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