Lanthanides contact relaxation

As we pointed out in Section 4.11, lanthanide shift reagents owe their utility partly to the fact that the electron spin-lattice relaxation time for the lanthanides is very short, so that NMR lines are not exceptionally broad. On the other hand, there are shiftless paramagnetic reagents that shorten both Tx and T2 to a moderate degree without causing contact or pseudocontact shifts. 

A good example of an investigation into the conformation of coordination of an organic ligand is the work of Singh, Reynolds and Sherry on lanthanide L-proline complexation. The H and paramagnetic shifts and relaxation rates were measured for all H and C atoms of aqueous proline-lanthanide ion solutions at pH 3 for ten lanthanides. It was concluded that while the relaxation data were consistent with an isostructural series, there were inconsistencies in the dipolar shift data, after separation from contact shifts, which could be resolved either by division into two, rather than one, isostructural series, or by invoking biaxial rather than uniaxial symmetry. 

Finally, the exchange lifetime of the water molecules, xy, is also a crucial parameter that affects the observed relaxivity. An excessively slow has a detrimental effect on rj (essentially because the effect of the paramagnetism is inefficiently passed to the bulk water), whereas a fast water exchange has the same effect on rj, as the water molecules are not in contact to the lanthanide ion long enough to efficiently experience its paramagnetic effect [15, 19, 25]. 

An alternative method of separating dipolar and contact effects of lanthanides has recently been proposed by Elgavish and Reuben (1978). The method involves measuring longitudinal relaxation rates under the assumption that these arise mainly from dipolar interactions. Since the relaxation rates for the different nuclei in a molecule depend on r , ratios of relaxation rates for various pairs of nuclei should be the same as iso-structural complexes. Such studies have provided evidence for isostructural sarcosine complexes along the lanthanide series. 

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