Fermi contact shifts electron spin densities from

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. 

This is a measure of the unpaired electron spin density that is transferred from the paramagnet to the nucleus of the spin under observation. In the regime defined by eq 2, the NMR shift (d = (Acolcoo)) induced by the Fermi-contact interaction is directly proportional to... 

The coupling of the unpaired electrons with the nucleus being observed generally results in a shift in resonance frequency that is referred to as a hyperfine isotropic or simply isotropic shift. This shift is usually dissected into two principal components. One, the hyperfine contact, Fermi contact or contact shift derives from a transfer of spin density from the unpaired electrons to the nucleus being observed. The other, the dipolar or pseudocontact shift, derives from a classical dipole-dipole interaction between the electron magnetic moment and the nuclear magnetic moment and is geometry dependent. 

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... 

The contact shift arises from delocalization of unpaired electron spin density, generally localized on a metal, to the ligand nuclei, and it depends inversely on temperature and directly on the Fermi contact spin density at the nucleus under NMR observation. The spin density, p /, represents the net imbalance between a and electron spins at the nucleus of interest positive and negative spin densities correspond to high- and low-frequency contact shifts, respectively. 

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. 

The chemical shift arising from interaction with the unpaired spin is a sum of two terms contact and dipolar (sometimes called pseudocontact). The Fermi contact interaction is the interaction between the nuclear spin and a net electron spin density at the nucleus, giving rise to a, the electron hyperfine interaction constant at the nucleus N, as observed in ESR experiments. The net electron spin density at the nucleus may arise directly when the unpaired electron wave function involves an s orbital centered on the nucleus, and indirectly when the closed s shells centered on the nucleus are spin-polarized by the unpaired electron in a cl or p orbital. 

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