|Nuclear Overhauser effect (NOE)|
When a specific nucleus is magnetically excited and its neighbor is at equilibrium, relaxation occurs between the two nuclei. Because of this relaxations, the intensity of the nucleaus that was at equilibrium changes. For a carbon and its neighboring proton it is possible to observe an almost four-fold increase in signal stranght resulting from decoupling. On the other hand in the 15N spectrum, the effect is negative and attenuates the 15N signal by 89%.
The smaller the physical distance between the equilibrium nucleus and the excited nucleaus, the bigger the expected change. The change for a carbon, two bonds away from the proton is about 3% by comparison to 297% for a single bond. This dependence can be used to estimate the distance between nuclei in a 2D experiment such as NOESY or ROESY.