Paramagnetic metal ions

The relaxation enhancement displayed for canthaxanthin, 7 -apo-7 -(4-carboxyphenyl)-P-carotene and BI was analyzed to provide interspin distances. The dipolar interactions and the distances can be determined according to procedures described elsewhere (Budker et al. 1995, Rakowsky et al. 1995, Eaton and Eaton 2000, Rao et al. 2000) and based on simulations of the paramagnetic metal ion contribution, WAA. 

ESR studies of other paramagnetic metal ions (Mn " ", VC)2+) as surface probes have confirmed that cation rotational mobility on fully wetted smectites with interlayer spacings of 1.0 nm or more is not more than 50% reduced relative to aqueous solution (12, 14). Apparently, about four molecular layers of water in the interlayer provide sufficient space for solution-like rotation rates of hydrated metal ions. 

PRE effects can also be seen for nuclear spins belonging to the solvent or other molecules present in solution, that essentially do not enter the first coordination sphere of the paramagnetic metal ion. This mechanism is... 

From all these considerations, it results that it is usually very easy to obtain information on the presence or absence of protons in the first coordination sphere of a paramagnetic metal ion from the analysis of solvent proton relaxivity, but it may be hard to obtain their number, or to have information on second coordination sphere protons. 

The classical equation for 7 sis provided in Section VII.A of Chapter 2. It depends only on the spin quantum number S, on the molar concentration of paramagnetic metal ions, on the distance d, and on a diffusion coefficient D, which is the sum of the diffusion coefficients of both the solvent molecule (Dj) and the paramagnetic complex (Dm), usually much smaller. The outer-sphere relaxivity calculated with this equation at room temperature and in pure water solution, by assuming d equal to 3 A, is shown in Pig. 25. It appears that the dispersions do not have the usual Lorentzian form. 

An electron spin can relax by coupling with a neighboring electron spin. Therefore, when a paramagnetic metal ion interacts with a second paramagnetic metal ion, the electron relaxation rates of the two metal ions may be dramatically affected. If Si and S2 are the two spins coupled by a scalar interaction, new spin levels will be established due to the interaction, with total S varying in unitary steps from Si — S2I to Si + S2. The energies of these spin levels are given by )... 

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