Nuclear Overhauser effect (NOE)

The nuclear Overhauser effect (NOE) causes changes in peak areas, as a secondary effect of decoupling. In most cases, it leads to an increase in areas, the maximum value of which is governed by the relative magnitudes of the magnetogyric ratios for the decoupled and observed nuclides:

NOE (max) = 1- g_dec/g_obs

For example, the g ratio for the ¹H-¹³C pair is about 4, so the maximum NOE that could be encountered is 3. Note that, for some nuclides, g < 0, leading to negative NOE's, which actually reduce signal intensity. The exact value of the NOE experienced by a particular nucleus in the sample depends on the number of directly attached decoupled nuclei, and on its spin-lattice relaxation characteristics. For ¹³C, methyl groups typically exhibit the largest NOE's, followed by methylenes, then methines. Quaternary carbons feel little effect. Because the enhancement is uneven, the NOE should be suppressed when quantitative results are desired. This can be accomplished by gating the decoupler, i.e., by turning it on only during the data acquisition time. This relatively brief period is sufficient to achieve decoupling, but does not provide enough time for the NOE to develop to any appreciable extent.