A Comparison of dipolar, contact, and Curie nuclear spin relaxation

8 A COMPARISON OF DIPOLAR, CONTACT, AND CURIE NUCLEAR SPIN RELAXATION 

Once R m and Rim have been measured, it is useful to try to understand the relative weight of dipolar, contact, and Curie spin contributions to the overall relaxation effect. Indeed, each of the three contributions is independently capable of providing valuable information whereas the whole value may not. The dipolar and Curie relaxation mechanisms — Eqs. (3.16), (3.17), (3.29) and (3.30) — 

A comparison of dipolar, contact, and Curie nuclear spin relaxation 

The presence of contact relaxation indicates that a given moiety is covalently bound to a paramagnetic metal ion and provides an estimate of the absolute value of A (Eqs. (3.26) and (3.27)). Sometimes the contact coupling constant can be evaluated by chemical shift measurements, and it is therefore possible to predict whether the contact relaxation contributions to R m, Rim or both, are negligible or sizable. 

Unlike hyperfine isotropic shifts, which often contain pseudocontact and contact contributions of the same order of magnitude, relaxation rates can often be recognized to be dominated by only one of the possible contributions. In addition, whereas contact and pseudocontact shifts may happen to have different signs, thereby making their separation more uncertain, relaxation contributions are obviously always positive and additive. 

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