Contact shift anisotropy

Contact shift anisotropy. A particular case of shift anisotropy is that stemming from the dipolar interaction between a nuclear spin and the spin density located outside the s core of a molecule. While the Fermi contribution coming from the electron spin density located directly at the nuclear site, that is, in the s orbital of the atom, is isotropic, the spin density located in the atomic p, d or f orbitals may produce a traceless anisotropic coupling to the nuclear spin. 

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. 

We should note that if g = ge, the contact shift is isotropic (independent of orientation). If g is different from ge and anisotropic (see Section 1.4), then the contact shift is also anisotropic. The anisotropy of the shift is due to the fact that (1) the energy spreading of the Zeeman levels is different for each orientation (see Fig. 1.16), and therefore the value of (Sz) will be orientation dependent and (2) the values of (5, A/s Sz S, Ms) of Eq. (1.31) are orientation dependent as the result of efficient spin-orbit coupling. On the contrary, the contact coupling constant A is a constant whose value does not depend on the molecular orientation. 

Let us suppose now that we have a solid with all molecules aligned with one another (Fig. 2.3) and that we perform the NMR experiment on a single crystal. If g of the S manifold equals ge, the contact shift contribution will be independent of the crystal orientation in the magnetic field. If, however, g has a different value in any k direction, then spin-orbit coupling is not negligible and the contact shift will be orientation dependent. Specific calculations are needed. If, however, we arbitrarily neglect the anisotropy of J/ Sz rlr), the following equation can be written... 

When the solid is dissolved in a liquid, the rotational rate of the molecules is fast with respect to the difference in hyperfine shift due to the electron Zeeman anisotropy, and the contact shift will be an average ... 

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)... 

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. 

The H NMR spectra of Co(ll)-substituted BCP are characterized by a large dispersion of signals that, nevertheless, is smaller than that observed in the native Cu(ll) proteins. All the hyperfine shifted signals can be detected directly due to the narrower linewidths (Fig. 4). The chemical shift range is not only due to differences in the electron delocalization (contact shifts), but also to the considerable magnetic anisotropy of the Co(ll) ion, even when tetracoordinated (Donaire et al., 1998). As a consequence, sizable pseudocontact shifts are induced on nuclei close to the metal ion, whether or not they belong to the metal ligands. 

Low-spin Fe(iii) porphyrins have been the subject of a number of studies. (638-650) The favourably short electronic spin-lattice relaxation time and appreciable anisotropic magnetic properties of low-spin Fe(iii) make it highly suited for NMR studies. Horrocks and Greenberg (638) have shown that both contact and dipolar shifts vary linearly with inverse temperature and have assessed the importance of second-order Zeeman (SOZ) effects and thermal population of excited states when evaluating the dipolar shifts in such systems. Estimation of dipolar shifts directly from g-tensor anisotropy without allowing for SOZ effects can lead to errors of up to 30% in either direction. Appreciable population of the excited orbital state(s) produces temperature dependent hyperfine splitting parameters. Such an explanation has been used to explain deviations between the measured and calculated shifts in bis-(l-methylimidazole) (641) and pyridine complexes (642) of ferriporphyrins. In the former complexes the contact shifts are considered to involve directly delocalized 7r-spin density... 

Multiple-pulse homodecoupUng combined with MAS Chemical shift anisotropy Contact time... 

To obtain electron-spin densities it is necessary to distinguish Fermi contact shifts from the pseudocontact shifts for each compound. This may be done sometimes by comparing two series of complexes differing only in central metal ion, if it can be shown that the modes of spin delocalisation are identical but one member is magnetically isotropic. Alternatively, the shifts for a given nucleus in the paramagnetic species are compared in solution and in the solid. Fermi contact shifts are the same in fixed and mobile phases the ratio of pseudocontact shifts in fixed and mobile environments is related to the g-value anisotropies. (While theoretically generally applicable the method is restricted because of the wide lines obtained with solids.) Discrimination between... 

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