Change of intensity of one resonance when the spin transitions of another are somehow perturbed from their equilibrium populations.
Notes: - The nuclear Overhauser effect at nucleus \(i\) for perturbation of spin \(S\) is expressed as a relative intensity change between the equilibrium intensity (\(I\)) and that in the presence of the nOe (\(I_{\rm{o}}\)). \(\eta_{i}({\rm{S}}) = (I - I_{o})/I_{o}\). The effect may be given as a percentage \(\eta \times \pu{100 \%}\).
- nOe allows a measure of through space rather than through bond interactions between nuclei and is proportional to the inverse \(6^{\rm{th}}\) power of the internuclear distance.
- \(\eta\) can be positive or negative depending on the motional properties of the molecule and the signs of the gyromagnetic ratios.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 698 (https://doi.org/10.1515/pac-2019-0203)