Separation of pseudocontact and contact contributions

The following method for separation of pseudocontact and contact shifts, proposed by Dobson et al. [83], is valid for axial systems assuming that (1) there is a nucleus in such a position in the molecule that the contact contribution is zero because it is far from the paramagnetic center and (2) the contact hyperfine coupling is constant ( 10%) for Nd-Tm. 

If 5 n = 0, then the ratio of the total hyperfine shifts 5hyp for nuclei i and j is 

By plotting 5 S0/5J° vs. gj(gj — 1 )J(J + 1 )/S J° for all the lanthanides, the intercept gives / , and hence the ratio between pairs of pseudocontact shifts (Fig. 2.22). Within this approximation, the requirement for axial symmetry was found to hold for many systems, even with low symmetry, possibly as a result of ligand rearrangement which might be fast on the NMR time scale. 

Separation of pseudocontact and contact shifts in axial systems can also be achieved, again by using a variety of lanthanides, without assuming that for a given signal the contact shift is zero, as long as the expected patterns of both pseudocontact and contact shifts (Fig. 2.21) hold. Here, the hyperfine shift can be expressed as the sum of contact and pseudocontact shifts (Eqs. (2.39) and (2.37))  

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