Paramagnetic contact shifts

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... 

Ion pairing between Li and 1 and Led was also investigated by analysis of the functional dependence of Li NMR chemical-shift values on the total concentrations of L15I and of le reduced Liblred (Fig. 5). Evidence for the formation of solvent-separated pairs was provided by the small value of the molar paramagnetic contact shift (8c) in Li NMR spectra of paramagnetic solutions of Lidlred (Fig. 5b). ... 

A study of labeled compounds derived from ethylcobalt(m) oetaethylporphyrin and other porphyrins indicated that the NMR resonances of the carbon atoms bound to cobalt were quite broad and were upheld shifted. The anomalous features of the NMR spectra were ascribed to the quadrupolar cobalt nucleus, and to the paramagnetic contact shifts that arose from thermal population ground state instead of agostic bond. ... 

On the other hand, if the chemical entity under consideration has a contribution of triplet or higher-spin configurations, its nmr signals are expected to suffer from contact shifts and broaden readily due to paramagnetic relaxation of the observing nuclear spins. It could be extremely difficult to observe nmr signals for such species under the above experimental conditions. 

Paramagnetic tons may alter NMR chemical shifts by what is known as a contact shift or by what is known as a pseudoconlact shift Consult an NMR book and explain II difference... 

In paramagnetic molecules with anisotropic g-tensors electron-proton dipole-dipole coupling may contribute to the hyperfine shifts observed in the proton NMR spectra. From the data to be discussed in this section it would seem, however, that in low spin ferric heme compounds many of the qualitative spectral features are mainly determined by Fermitype contact shifts. 

The paramagnetic properties of Co(II) have been utilized in some biochemical applications of nuclear magnetic resonance. Cobalt(II)-induced contact shifts were observed in lysozyme (26). A preferential binding of Co2+ to a single site presumably involving two carboxyl groups was deduced. This technique might become very informative in studies of metal ion-dependent enzyme systems. 

The following method for separation of pseudocontact and contact shifts, proposed by Dobson et al. [83], is valid for axial systems assuming that (1) there is a nucleus in such a position in the molecule that the contact contribution is zero because it is far from the paramagnetic center and (2) the contact hyperfine coupling is constant ( 10%) for Nd-Tm. 

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

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

B-NMR shift, 8 19.6. For the paramagnetic neutral and cationic dinu-clear complexes, no ESR signals have been obtained. However, in the H-NMR spectrum of the paramagnetic neutral compounds, typical paramagnetic shifts occur. As the 33-valence-electron triple-decker 29 is easily reduced to its diamagnetic anion, the contact shifts have been studied by successive reduction of 29 with potassium (39). 

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. 

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