What is Nuclear Overhauser effect?

Saturating or perturbing one spin changes, through their dipolar relaxation, the intensity of the signal of another spin close in space: the nuclear Overhauser effect (NOE). Its build-up rate is proportional to $r^{-6}$, so it is seen only between nuclei less than about $0.5\,\mathrm{nm}$ apart, whether bonded or not.