Effects of Paramagnetic Substances

The chemical shift arising from interaction with the unpaired spin is a sum of two terms contact and dipolar (sometimes called pseudocontact). The Fermi contact interaction is the interaction between the nuclear spin and a net electron spin density at the nucleus, giving rise to a, the electron hyperfine interaction constant at the nucleus N, as observed in ESR experiments. The net electron spin density at the nucleus may arise directly when the unpaired electron wave function involves an s orbital centered on the nucleus, and indirectly when the closed s shells centered on the nucleus are spin-polarized by the unpaired electron in a cl or p orbital. 

When the electron spin relaxation time 7, is sufficiently short that, for a j in joules, 

This assumes that the magnetic moment of the paramagnetic species contains no orbital contribution except that produced by spin-orbit coupling and that there is only one thermally accessible paramagnetic state with spin S. These conditions are not strictly true for some transition metal complexes. 

The dipolar shift arises from the direct magnetic interaction of the electron spin and the nuclear spin. For molecules with tetragonal symmetry, the dipolar shift is  

There are some unwanted effects of paramagnetic additives. They may wash out spin-spin coupling, either by too fast relaxation (for small /), or by promoting decoupling by fast exchange. The magnitudes of coupling constants may be altered. 

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In the NMR investigation of solutions, the effects of paramagnetic substances in solution on the proton resonance peaks of the solvent may be of particular significance. Certain europium(III) and praseodymium(III) chelates, for example, behave as Lewis acids towards donor solvents such as alcohols, ethers, esters and ketones. As a result of this acid-base interaction the proton resonance peaks undergo shifts which favour, among others, the resolution of the PMR spectrum. 

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