Electron Spin-Nucleus Interactions and Consequences

Acceleration of relaxation of a nuclear spin by paramagnetic compounds is the greater the slower the nucleus relaxes. For example, the nonprotonated carbon nuclei of 

The pronounced relaxation acceleration observed for slowly relaxing nuclei in the presence of paramagnetic compounds is exploited in Fourier transform, 3C NMR spectroscopy. On use of the fast pulse sequences that are frequently necessary, the spin-lattice relaxation of slow 13C nuclei can no longer follow excitation, and the corresponding 13C signals have low intensities. In such cases, addition of small amounts of relaxation accelerators, such as radicals or transition metal salts, to the sample amplifies these signals [153]. 

It ist quite reasonable that a paramagnetic central ion should accelerate the relaxation of the C atoms of a ligand, and that the magnitude of the acceleration should depend upon the distance between the ion and the C atom being observed. 

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