Nuclear Overhauser Effect:
The Nuclear Overhauser Effect was first described by Overhauser as the technique by which the intensity of NMR signals can be enhanced significantly by irradiation of some of the nearly nuclei within the molecule. This is illustrated as follows-
Consider two different protons, HA and HB in the hypothetical molecule (I), which are too close to allow any through-space interaction of their fluctuating magnetic vectors; each can contribute to the other’s spin-lattice relaxation process (T1). The number of intervening bonds between HA and HB is too large to allow any through-bond coupling between them.
The spectrum of the compound is run and integrated. It is then run again with a relatively low power decoupler irradiating HB and reintegrated. If HA and HB are relatively closer in space (i.e., not more than about 4.5Å apart), then the intensity of HA will be somewhat enhanced in the second spectrum. If HA and HB are separated by more than about 4.5Å, then no such enhancement will be observed.
If one equation records the 1HNMR of isovanillin normally and then while irridating at the CH3O frequency, the integral for the ortho proton (doublet) is markedly increased i.e., this doublet then appears as a more intense signal.
As a second example, if one irradiates the vinyl hydrogen in 3-methyl but-2-ene-1-oic acid and record the integral while doing so, the integral of the cis-methyl group will be larger than that of the trans-methyl group. This method is useful to determine stereochemical relationships in molecules. On irradiation of the cis-methyl group (a), HC shows a 17 per cent increase in its integral intensity. Irradiation of the more remote trans methyl group (b) does not affect the intensity of the integral of HC.