Pseudo contact interaction

In general, induced shifts Ad may arise by contact interaction (through-bond interaction) or by dipolar (pseudo-contact) interaction (through-space interaction)87. In rare earth ions the inner/electrons are well shielded and do not participate in covalent bonding (contact interaction). Thus induced shifts originate mainly from pseudo-contact interactions. 

Y2- LtijTi207 are one order of magnitude smaller and are best explained in terms of the pseudo-contact interaction mechanism [52]. 

Equation (5.14) amounts to a description of the anisotropic coupling A°° + A originating from the interaction of an electron spin magnetic moment i = , UsgS with the proton magnetic moment The isotropic part of gT which is usually called the pseudo contact interaction is only zero for isotropic g tensors. Since in (5.14) the g tensor is known from EPR data, the remaining parameters which describe A are a, and the direction cosines, nj. 

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. 

Pseudo-contact shifts and through-space interactions 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)... 

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. 

Figure 1.10. (a) Second-order spin-rotation interaction occurring via L. (b) Second-order pseudo-contact hyperfine interaction occurring via L. (c) Electron coupled nuclear spin-spin interaction, (d) Second-order interaction of R and L. 

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. 

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-... 

The biochemical structure of yeast TBP bound to DNA has been determined by X-ray crystallography and two very interesting observations have been made. The first is that TBP has an internal pseudo-dimer interaction between two adjacent protein domains, forming a "saddle" structure that binds DNA (Figure 24. 7). The exposed surface of TBP in this configuration serves as a contact area for a variety of other transcription factors. 

Nuclear magnetic resonance spectra of these compounds are highly informative in this regard. The lineshapes of the Sn and resonances are dominated by the coupling of the nuclei with the unpaired electron spins. This interaction results in large resonance shifts arising from both the Fermi contact and the pseudo-contact (dipolar) interaction. 

In paramagnetic systems NMR shift differences may become quite considerable due to pseudo oontaot and/or contact interactions (cf. 

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