Pseudocontact shift magnetic susceptibility

Eq. (2.20), or its simplified version in the axial case, Eq. (2.18), are of general validity. However, the principal directions and components of the molecular X tensor are seldom available. Pseudocontact shifts can be still evaluated by expressing the principal molecular magnetic susceptibility values as a function of the principal g values, in analogy with Eq. (1.38) ... 

Pseudocontact shift is expected every time there are energy levels close to the ground state. This causes orbital contributions to the ground state, and such contributions are orientation dependent. Therefore, the magnetic susceptibility tensor is anisotropic. Anisotropy of the magnetic susceptibility tensors arises also from sizable ZFS of the S manifold (Section 1.4). 

Pseudocontact shifts are estimated using single crystal magnetic susceptibility data. 

This is not in accord with theory for pure 7r-delocalization between metal and ligand which predicts that a-contact shifts for H and be of opposite sign. However, possible large pseudocontact shifts in the low spin state of these complexes complicate the issue. For a series of Fe(dtc)2X complexes (X = Cl, Br, I), however, accurate estimates of pseudocontact shifts from magnetic susceptibility data can be made. 

Fischer has proposed useful and important methods for factoring the isotropic shifts of uranocenes into contact and pseudocontact components (15) values were reported for uranocene, 1,-1, 3,3, 5,5, 7,7 -octamethyluranocene, and 1 1 -bis(trimethyl-si lyl) uranocene using a non-zero value of Xj Fischer arrived at values of yjj2 and y 2 at several temperatures from the ratio of the geometry factor and the isotropic shift for methyl protons in bis(trimethylsilyl)-uranocene, and bulk magnetic susceptibility data, assuming no contact contributions to the isotropic shift of the methyl protons. From the published data of Fischer, the value of y( - y2 at 30°C is 8.78 BM2. 

Lanthanide ions with anisotropic molecular magnetic susceptibility tensor Ay, for example, Er , Tm , or Yb , allow valuable structural information to be extracted from pseudocontact shifts (PCS, see also Section 3.6) and residual dipolar couplings (RDCs) arising from the partial alignment of the -labeled biomolecules in the induced magnetic field (Su et al., 2008b) ... 

These complexes, which either are salts, or at least contain coordinated anionic ligands in which case a large Ln-ligand bond moment should result from the rather electrostatic nature of the bond, tend to be insoluble in solvents other than polar solvents in which they dissociate. Hence physicochemical studies in solution are limited. However, solid state fluorescence spectra of phenanthroline and bipyridyl complexes of Eu have been obtained and correlated with the molecular site symmetry (for a discussion of this tyrc of fluorescence, see Section 39.2.7.2). Magnetic susceptibility values have been measured for the series of complexes M(N03)3(phen)2 at 293 K and are (BM) as foUows La, 0 Ce, 2.46 Pr, 3.48 Nd, 3.44 Sm, 1.64 Eu, 3.36 Gd, 7.97 Tb, 9.81 Dy, 10.6 Ho, 10.7 Er, 9.46 Tm, 7.51 Yb, 4.47 Lu, 0. These values correspond closely with the calculated values. The complexes Ln(N03)3(4,4-di- -butyl-2,2 -bipyridyl)2 and Ln(N03)3(5,5 -di-n-butyl-2,2 -bipyridyl)2 are soluble in organic solvents and give NMR spectra which show paramagnetic shifts. These were at first believed to be of a covalently induced, or contact, nature but it later became evident that they incorporate a substantial dipolar, or pseudocontact, component also. Electronic spectra of these complexes were also obtained in solution. They showed only a small nephelauxetic effect with =0.96, which indicated very little covalent character in the Ln—bond. 

Valencia et have used proton pseudocontact chemical shift data in conjuction with x-ray crystal structures to determine the solution geometries of the lanthanide complexes with the ligand Py2N6Ac4, which contains four acetate pendant arms. The aqueous solution structures of diamagneticLa and Lu complexes were characterized by their COSY NMR spectra. The structures of the paramagnetic complexes were determined by fits of H pseudocontact chemical shift data and spin relaxation data to a model that assumed rhombic magnetic susceptibility tensors. 

Figure 6.23 Stereoview of the Cu(II)-CopC structure family calculated using NMR constraints and information derived from EXAFS data. Pseudocontact shifts are depicted on the protein frame as spheres. Examples of positive (P) and negative (N) spheres are shown. The radius of each sphere is proportional to the absolute value of the pseudocontact shift. The principal axis of the magnetic susceptibility tensor is also shown (axis). 2003 American Chemical Society.

Fig. 2 Spatial dependence of the dipolar shift anisotropy (a) and of the pseudocontact shift in the magnetic susceptibility principal axis system (b). The position of the nucleus is defined by its spherical coordinates (r, 6, tp) in the PAS of the x tensor. Violet surfaces represent isosurfaces of Aa, and blue and red surfaces represent respectively positive and negative isosurfaces of...

Bertini et report a detailed study of the magnetic properties of myglobin, in which they determined the axial and orthorhombic terms of the paramagnetic susceptibility tensor using a combination of hyperfine chemical shift measurements and static suscetibility measurements (Evans Method). The determination of the magnetic anisotropy provided a measurement of the residual dipolar couplings and also permitted a separation of the contact and pseudocontact chemical shift contributions of resonances of the Fe(III) ligands. 

---