Influence of Paramagnetic Species on the Internuclear Overhauser Effect

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Abstract
It is shown that internuclear Overhauser enhancements can be substantially reduced if a paramagnetic species is added to the sample. Such reduction or elimination of the Overhauser enhancement can be accomplished without significant broadening of a nuclear resonance, if the nuclei observed are embedded within a molecule and those decoupled are in the periphery. Thereby the intensity information and resolution can be retained in proton decoupled 13C spectra of molecules in solution. Also, the method should be applicable to decoupled spectra of species such as B, N, Si, and P. In cases where Overhauser enhancements are not reduced to zero, the actual magnitude of the enhancement can be calculated from a plot of signal intensity versus concentration of paramagnetic species. Characteristic spin‐lattice relaxation times can also be inferred from such a plot. The theory is extended to multiple spin systems and to cases of complex formation, and it is shown that preferential reduction of Overhauser enhancements by a paramagnetic complexing species offers an extremely sensitive means for determining ligand binding sites and for inferring paramagnetic‐diamagnetic spin separations. Observation of transient nuclear Overhauser effects in both the presence and absence of added paramagnetic material can also be used to determine the absolute magnitude of Overhauser enhancements and nuclear relaxation times.