Pseudo-contact shift

With these assignments at hand the analysis of the hyperfine shifts became possible. An Fe(III) in tetrahedral structures of iron-sulfur proteins has a high-spin electronic structure, with negligible magnetic anisotropy. The hyperfine shifts of the protons influenced by the Fe(III) are essentially Fermi contact in origin 21, 22). An Fe(II), on the other hand, has four unpaired electrons and there may be some magnetic anisotropy, giving rise to pseudo-contact shifts. In addition, there is a quintet state at a few hundred cm which may complicate the analysis of hyperfine shifts, but the main contribution to hyperfine shifts is still from the contact shifts 21, 22). 

Instead of measuring only the time-dependent dipolar interaction via NOE, it is also possible to determine dipolar couplings directly if the solute molecule is partially aligned in so-called alignment media. The most important resulting anisotropic parameters are RDCs, but residual quadrupolar couplings (RQCs), residual chemical shift anisotropy (RCSA) and pseudo-contact shifts (PCSs) can also be used for structure determination if applicable. 

No significant pseudo-contact shifts could be induced in the spectra of dibenzothiophene, its sulfoxide, or its sulfone, with Eu(dpm)3 although a marked shielding of the protons of one ring was observed in the spectrum of the chromium tricarbonyl complex of dibenzothiophene,presumably due to a contact shift mechanism. 

Pseudo-contact shifts and through-space tic acid) 381... 

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... 

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)... 

In the principal magnetic axes system, the magnetic susceptibility tensor is diagonal and the three last terms of eq. (27) vanish, thus leading to eq. (28) for the pseudo-contact shifts... 

Equations (27)—(29) imply that pseudo-contact shifts Sfc are maximum for complexes displaying large molecular magnetic anisotropies and that structural and geometrical informations can be extracted from the so-called non-linear geometrical factors G (eq. (30)) and Hi (eq. (31)) (Forsberg, 1996 Peters et al., 1996)... 

The first numerical terms C = P2 1 + p) j / (l20(kT)2) are often referred to as Bleaney s factors and their relative values (scaled to Coy = -100) have been tabulated at 300 K for any 4f configurations including excited states contributions (table 3, Bleaney et al. (1972)). Finally, the introduction of the geometrical factors defined by eqs. (30), (31) together with C into eq. (41) gives the classical eq. (42) for the pseudo-contact shifts according to Bleaney s approach (Forsberg, 1996)... 

RDCs belong to the so-called anisotropic NMR parameters which cannot be observed in isotropically averaged samples as, for example, is the case in liquids. Besides RDCs, a number of other anisotropic parameters can be used for structure elucidation, like residual chemical shift anisotropy, residual quad-rupolar couplings for spin-1 nuclei, or pseudo-contact shifts in paramagnetic samples. Here, we will focus on RDCs where we give a brief introduction into the dipolar interaction, then into the averaging effects with the description by the alignment tensor and concepts to deal with the flexibility of molecules. For the other anisotropic NMR parameters, we refer the reader to ref 19 for an introduction and to refs. 6-8 for a detailed description. 

Trivalent gadolinium with f7 configuration has isotropic distribution of electrons and hence cannot produce pseudo contact shift. However, when the Lewis acid-base interaction is partly covalent, the unpaired electron spin density influences the molecular framework of the base and causes an LIS known as contact shift. Gd(III) is used to ascertain the contributions of contact shift to the measured LIS. 

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. 

If the complex is paramagnetic, then the chemical shift can occur over a much wider range (see Table XVIII). These large chemical shifts arise from either contact shifts due to delocalization of unpaired electron density from the metal to the ligand or from pseudo contact shifts due to the magnetic field produced by the unpaired electron density. The interpretation of such shifts is generally difficult, but a sensitive criterion for TT-delocalization using methyl substituents has been proposed on the basis of theoretical considerations (66), namely Eq. (5)... 

Pseudo-contact shifts can arise only in systems where the magnetic susceptibility of the paramagnetic center is anisotropic. This interaction appears to be dominant in certain transition-metal ions (Co ), as well as such rare-earth elements US Dy , Er +, and Yb. The chemi( shift from the dipo-... 

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