Paramagnetic anisotropy

Rotational g-values, magnetic susceptibilities and anisotropies, paramagnetic and diamagnetic contribntions, molecular quadrupole moments, electronic charge distributions, spin-rotation and spin-spin coupling parameters, nuclear g-values from the rotational Zeeman effect and nuclear magnetic shieldii parameters from the rotational Zeeman effect as well as indirectly via the paramagnetic contribution to the... 

In contrast, soft magnetic solids and paramagnetic systems with weak anisotropy may be completely polarized by an applied field, that is, the effective field at the Mossbauer nucleus is along the direction of the applied field, whereas the EFG is powder-distributed as in the case of crystallites or molecules. In this case, first-order quadrupole shifts cannot be observed in the magnetic Mossbauer spectra because they are symmetrically smeared out around the unperturbed positions of hyperfine fines, as given by the powder average of EQ mj, d, in (4.51). The result is a symmetric broadening of all hyperfine fines (however, distinct asymmetries arise if the first-order condition is violated). 

In principle it should be possible to determine the anisotropy of the paramagnetic susceptibility for some systems of pseudo-axial symmetry, but apart from the citation (74) of a private communication regarding the ferricenium cation, no experimental data are available for metal sandwich complexes. Such measurements should however be possible for at least some metallocenes and mixed sandwich complexes since these are found to crystallise in either a monoclinic (Fe(Cp)2 (6)) or orthorhombic (Ru(Cp)2, (Cp)V(Ch),... 

In metalloproteins, the paramagnet is an inseparable part of the native biomacromolecule, and so anisotropy in the metal EPR is not averaged away in aqueous solution at ambient temperatures. This opens the way to study metalloprotein EPR under conditions that would seem to approach those of the physiology of the cell more closely than when using frozen aqueous solutions. Still the number of papers describing metalloprotein bioEPR studies in the frozen state by far outnumbers studies in the liquid state. Several additional theoretical and practical problems are related to the latter (1) increased spin-lattice relaxation rate, (2) (bio)chemical reactivity, (3) unfavorable Boltzmann distributions, (4) limited tumbling rates, and (5) undefined g-strain. 

In metalloproteins two paramagnets can be much farther apart, and so the dipolar interaction can be correspondingly weaker. Furthermore, the centers will usually each have significant g-anisotropy, and their local structures will differ and will have a complex mutual geometrical relationship. We therefore use the symmetric biradical as a simple model to obtain a first impression of the type of spectral patterns to be encountered. 

Of the lanthanide (III) ions, gadolinium(III) is exceptional, being the only isotropic paramagnetic ion of this series. Anisotropy, D, in 3d systems has been shown to hinder a large magnetocaloric effect at low temperatures as demonstrated by... 

As another example, the three-dimensional structure of Cytochrome c has been determined on the basis of structural information from pseudocontact paramagnetic chemical shifts, Curie-Dipolar cross-correlation, secondary structure constraints, dipolar couplings and 15N relaxation data [103]. This protein has a paramagnetic center, and therefore the above-mentioned conformational restraints can be derived from this feature. Dipolar couplings do not average to zero because of the susceptibility tensor anisotropy of the protein. The structure determination of this protein without NOE data gives an RMSD (root... 

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