Lanthanide-induced shifts separation shift contributions

Quantitative separation of the n contact and pseudocontact contributions to the lanthanide induced shifts (LIS) in aniline and m-and p-toluidines has been reported. (395, 396) The contact shift patterns are estimated from the rr-spin density distribution of the appropriate cation radical or from the Ni(acac)2 induced shifts. The separation of the shifts was checked by comparing the relative contact shift contribution with the <5 ) value of Golding and Halton (389) and the remaining pseudocontact contribution with the calculated values of Bleaney s theory. (380)... 

Solution structures may be determined on the basis of lanthanide-induced shifts. This is, however, a difficult task the exact origin of the induced NMR shifts is unknown, a great number of lanthanide complexes have a low symmetry and they often present a high lability, e.g., between conformers. The separation of the various contributions to the NMR shifts may be achieved by several procedures under favourable conditions, or can be avoided by selecting the Yb(III) ion as paramagnetic center, since this ion induces shifts that are essentially dipolar in origin. The dipolar shift is expressed by ... 

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