The paramagnetic shift

The interactions of an electron and a magnetic nucleus with an applied magnetic field and the interaction between the electron and the nucleus may be represented by the total spin Hamiltonian  

The first two terms are Zeeman terms and the third represents the hyperfine interaction of the electron and nuclear spins, / b and are the Bohr and nuclear magnetons respectively, S is a fictitious effective spin (S = 2 for a simple Kramers doublet), and / is the nuclear spin tensor. The hyperfine tensor is further split into Fermi contact, dipolar, and orbital components according to  

In equation (3) the Dirac delta function 5(r,) expresses the probability density for the electron i at the nucleus. Since 

The equations for other solution averaging conditions are given in references (18) and (19). In the special case of a spin-only state with an isotropic g-tensor (i.e. gy = gi and g n = gsi = 2) equation (9) reduces 

For the d octahedral case, the above expressions require a change in sign of the spin-orbit coupling constant The authors also derived the 

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Figure 3. Monitoring the uptake of Li+ into human erythrocytes after incubation in media containing 2 mM Li+ using 7Li NMR spectroscopy. The signals corresponding to the intra-, and extracellular Li+ are separated by the presence of the paramagnetic shift reagent, Dy(P30io)2, the extracellular medium [34].

NMR has also been used to characterize binding [22, 23]. When cyt c binds to ccp the paramagnetically shifted heme methyl groups of cyt c shift further downfield by 1-2 ppm. Normally, binding occurs in the fast exchange regime, so that the fraction bound can be assessed from the frequency shift (Fig. 10). Two... 

Magnetic susceptibilities of solutions — These are useful parameters for determining equilibrium constants for reactions involving spin changes. The Evans nmr method utilizes the observed shift in the resonance line (say of a proton of t-BuOH or hexamethyldisilox-ane) in solution when a paramagnetic substance is added. The paramagnetic shift A/ is related to the magnetic moment (jj, of the solution at TK by the approximate expression... 

Lanthanide Shift Reagents.—New theoretical treatments of the origin of the paramagnetic shifts induced in the n.m.r. spectra of substrates bound to... 

For phosphorus involving tetra-, penta- or hexa- coordination the importance of d electrons (even if quantitatively small) must not be underestimated <1968,5) it is especially easy to detect as it strongly decreases the paramagnetic shift from which a net high-field shift arises. To consider these -contributions on a theoretical basis is rather complex<1964,1) and is at present limited to qualitative conclusions/1968,5) The case of the assumed PBr4Q ion, for which a shift of — 150 p.p.m. is observed (compared with —225 for PBr3) is rather interesting in this respect/1969 111... 

For high-spin ferric porphyrin with axial symmetry, the paramagnetic shift, (AH/H) is given by Eq. (2) [3] ... 

Fig. 6a, b. Temperature variation of the paramagnetic shift of ring methyl proton resonances of (a) diaqua hemin, and (b) aqua hydroxo hemin complexes, in 5% aqueous SDS micelles. (Taken from Ref. 20)... 

The effect of the micelles on the paramagnetic shifts of the heme was very clearly demonstrated [22] in NMR of labelled cyanide in [Fe(PP)(Ci"N)2]- and [Fe(PP)(py)(Ci N)] in different micelles as well as in the absence of micelles (Fig. 9). A pronounced systematic downfield shift of the bound cyanide signals is observed on going from a solution without micelles to SDS, to TX-lOO and to CTAB micellar solutions which is also the trend in increasing hydrophobicity. The signal is known to be extremely sensitive... 

It has been stated above that for a series of isostructural lanthanide complexes the paramagnetic shifts of analogous nuclei should be proportional to the parameter A, if a purely dipolar mechanism operates. The paramagnetic shift is usually taken as the difference between the chemical shift of the nucleus in the paramagnetic complex under investigation, and the... 

The effect of this equation is that the ratios of the paramagnetic contributions to the relaxation rates of a set of, for example, H nuclei belonging to the same complex are determined by the lanthanide 3H distance only. Hence a similar type of computer optimization as for the paramagnetic shifts may be used to test compatibility with hypothetical structures which incorporate distance-dependent parameters such as the orientation or conformation of an organic ligand. 

In addition to line broadening due to accelerated relaxation, paramagnetic salts and chelates give rise to isotropic shifts when added to samples containing molecules with groups susceptible to coordination with metal ions, e.g. —OH, —NH2, — SH, — COOH, -C = 0. The isotropic shift A of a nucleus i in a sample S is the difference between chemical shifts measured before and after addition of the paramagnetic shift reagent SR [103] ... 

Case II occurs when the transverse relaxation rate is faster than both exchange rate and paramagnetic shift. Here the observed shift is directly proportional to the paramagnetic shift. 

Thirdly, related plots according to eq. (68) involving H4 clearly exhibit two straight lines, one for the large (Ce-Tb) and one for the small (Er-Yb) lanthanide ions with an abrupt transition between them (fig. 57b). This behaviour contrasts with that observed for Hl-3,6,7 and apparently leads to opposite conclusions in which two different structural arrangements should be invoked. However, both metallic centres significantly contribute to the paramagnetic shift... 

Some of the polycrystalline spin crossover systems of iron(II) described above retain their spin equilibrium property upon dissolution in appropriate solvents. The Evans NMR method of measuring the change of the paramagnetic shift with temperature is the most common technique to study the magnetic behaviour of such systems. The spin transition characteristics has been observed to depend on various chemical modi-... 

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