Homo-nuclear dipolar coupling

When performing 2D-NMR experiments one must keep in mind that the second frequency dimension (Fx) is digitized by the number of tx increments. Therefore, it is important to consider the amount of spectral resolution that is needed to resolve the correlations of interest. In the first dimension (F2), the resolution is independent of time relative to F. The only requirement for F2 is that the necessary number of scans is obtained to allow appropriate signal averaging to obtain the desired S/N. These two parameters, the number of scans acquired per tx increment and the total number of tx increments, are what dictate the amount of time required to acquire the full 2D-data matrix. 2D-homo-nuclear spectroscopy can be summarized by three different interactions, namely scalar coupling, dipolar coupling and exchange processes. 

These enable intense homo- and hetero-nuclear coherence and polarisation transfers to take place, not to mention cross-relaxation (polarisation transfer under the influence of dipolar coupling) (Figure 5.19). Usually, the following sequence of operations is carried out. 

Homo-nuclear (e.g. H- H) correlation (COSY), heteronuclear ( H-X, X = 5n, F, 0,3 P) correlation (HETCOR) through- space and/or -bond dipolar coupling and nuclear interactions recorded by designed rf pulse sequences SS-NMR relaxometry... 

So far we have discussed the NOE between nuclei having the same y. The maximum use of these homo-nuclear NOEs has been for proton-proton NOEs. However, the heteronuclear NOE also has a long history. In the early days of NMR, the heteronuclear NOE was basically utilized for the enhancement of the intensities of low y nuclei. Eor example, the steady state NOE, on spin I on irradiation of spin S when spins I and S are mutually dipolar-relaxation coupled is given by Equations [4] and [6]. For cor, l, Wo Wi W2 2 3 12, a = yielding... 

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