Fermi contact shifts

The predictions of the proton shifts is difficult the prediction is even more difficult in the case of heteroatoms. In the case of cr spin density like in a aliphatic amine (Table 2.2A), the 13C Fermi contact shift for the a-carbon of the aliphatic amine is upfield (A/h is negative) because of predominant spin polarization effects, whereas that for other carbon atoms is downfield, and rapidly attenuates with the number of bonds. A sizable downfield shift is experienced by the 14N nucleus when nitrogen is a donor atom. 

For 19F Fermi contact shift of fluorine bound to sp2 carbon atoms, spin density on the nucleus arises from spin polarization by as for analogous CC moieties, and from spin polarization by /Op, which occurs via direct delocalization through C—F ji bonding (Fig. 2.18). The hyperfine coupling is therefore... 

The HNMR spectra of [Os(bipy)3]w and of [Os(phen)3]3+ have been measured and contact, pseudo-contact and Fermi-contact shifts studied 118149,153 the ESR spectra were also measured.149 The l3C resonance spectra of [Os(bipy)3]2+, [Os(4,4-Cl2bipy)3]2+, [Os(4,4-(OEt)2bipy)3]2+, [Os(4-OMebipy)3]2+ and [Os(4-NMe2bipy)3]2+ have been recorded.153 The resonance Raman spectrum of [Os(bipy)3j2+ has been measured.118... 

The Fermi contact shift describes the influence of the unpaired electron spin on nnclear chemical shifts as a resnlt of throngh-bond hyperfine conpling. The contact shift is caused by the presence of unpaired electron spin density at the observed nnclens. Thns, spin density must be transferred to an s orbital of the nnclens of interest, which is typically achieved through spin polarization. In the case of a single, isolated spin state for a molecule in solution, contact shift can be described by... 

Fermi contact shifts may also contribute to the hyperfine shift observed in n.m.r. spectra. These shifts arise from the delocalization of electron spin density from the extended orbital of the metal ion to the orbitals of the ligand. This shift is primarily dependent on the contact interaction constant for a given nucleus. Contact shifts yield no distance information or structural information. 

The Fermi contact shift term comes about from unpaired spin density transferred to the nuclei via bonding orbitals as well as screening and polarization effects. This contribution is isotropic and calculated according to Ato,... 

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... 

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... 

Pseudocontact or dipolar shifts are the results of dipolar interaction between the electronic magnetic moment and the nuclear spin which do not vanish in magnetically anisotropic systems. Like the Fermi contact shift their magnitude is proportional to and the distance between the two spins. 

Fermi-contact shifts and was further verified by comparison of NMR spectra of similar derivatives [25]. A solid-state molecular structure of 14a verified the interaction of the Lewis acid B(CgF5)3 with the distal nitrogen atom. 

It follows from these physical interpretations that the Fermi contact shift 8 reports on the spin density distribution, while long-range structural constraints can be deduced from analysis of the pseudocontact shift 8. ... 

The isotropic Fermi contact shift 5 in parts per million resulting from this interaction is... 

Fig. 13 (a, b) MAS NMR spectra of two nitronylnitroxide radicals (spinning rates of 10 kHz and 15 kHz, respectively), together with the corresponding sign patterns of the spm densities experimentally derived from the Fermi contact shifts up-spins indicate positive spin), (e) Side view of an unsubstituted nitronylnitroxide down from the center of the bond C4-C5 to C2. The spin-carrying MO is located on the NO groups. Reprinted with permission from [51], (C) 1999 American Chemical Society... 

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