NUCLEAR OVERHAUSER EFFECT (N.O.E)

The nuclear overhauser effect is, can be used to demonstrating those two protons or group of protons are in closeproximity within the molecules, and is therefore of considerable value in the study of molecular geometry.

                               The basic observation of (N. O. E) can be described by the reference to the hypothetical molecule I. The two protons Hₐ and Hb are imagined their called " FLUCTUATING MAGNETIC VECTORS"  each vectors can contribute to other's spin - lattice relaxation process. The to total number of intervening bonds between Ha and Hb is very too large to allow the normal coupling between them, but this is not a prerequisite for the operation of (N. O. E)

If we double irradiate at the Hb signals, we can stimulating absorption and emission processes for Hb and it's stimulation will be transferred through space to the relaxation mechanisms of Ha. The spin - lattice relaxation of Ha will be speeded up, leading to a net increase in the absorption signal of Ha. 

In this summary, provide that Hb makes a significant contribution to the spin - lattice relaxation process of Ha, then double irradiation of Hb of Hb causes increase in the intensity of the Ha signal (by 15 to 50%).

Since molecular oxygen is paramagnetic, and it can contributing to nuclear spin- lattice relaxation process, the (N. O. E) experiments should be carried out on deoxygenated samples to avoiding this possible complications. 

The simple examples of (N. O. E) is found in isovanillin. If we first record the nmr spectroscopy for isovanillin normally, and then while irradiating at the CH₃O frequency, the integral for the ortho proton which appears as a doublet is markedly increased.