The NOE Through-Space Interactions between Protons

NOE stands for Nuclear Overhauser Effect. Probably only physicists understand the NOE fully, and we shall not go into the theory but only present the results. It is a phenomenon which is useful and important in the NMR of both small and large molecules. 

The NOE depends on a special kind of relaxation known as dipole-dipole cross-relaxation. When one signal in an NMR spectrum is irradiated, the intensities of others may change this is called the NOE and its importance is due to the fact that the signals which react are due to spins which are physically close to that perturbed by irradiation. 

These remarks only apply to the proton-proton NOE experiments involving an NOE between the proton and another nucleus can also be carried out, and the NOE also has an effect on certain carbon-13 spectra, as we shall see later. 

Theory tells us that the maximum gain in proton signal intensity is 50%, but normally we are dealing with changes of only a few per cent, and the magnitude of these is dependent on the distance between the irradiated proton(s) and the observed ones the effect is too small to be visible when this distance exceeds about 5 A. 

The NOE is really quite complicated, and in fact even small molecules can show negative NOEs, which are due to a phenomenon known as spin diffusion. 

---