Beyond 2D spectroscopy

In a chapter regarding 2D NMR spectroscopy of paramagnetic molecules, the obvious perspective is that of using three-dimensional (3D) NMR for paramagnetic molecules. The demand for 3D spectroscopy is based on a need of increased resolution when macromolecules are concerned. It is possible that for small complexes 3D spectroscopy will never be necessary. However, every time something new has appeared in science, the majority has reacted by saying that the utility was scarce in their own field, and the majority has not always been right. Therefore, we do not commit ourselves. 

An obvious extension to 3D spectroscopy from 2D spectroscopy is the homonu-clear NOESY-NOESY [34]. There are two t variable times and one tj, which after Fourier transform provide three frequency domains. The 3D NOESY-NOESY spectrum of met-myoglobin cyanide, which contains low spin iron(III) in a heme moiety (see Fig. 5.7), has been successfully measured [35]. In Fig. 8.22 a slice of the 3D spectrum is shown at the I2-CH3 height. On the diagonal it shows all the dipolar connectivities between I2-CH3 and other protons off-diagonal 

Results similar to those shown in the slice of Fig. 8.22 can be obtained with the so-called NOE-NOESY sequence [36]. Here a hyperfine shifted signal, e.g. I2-CH3 of the above compound, is selectively saturated, and then the NOESY pulse sequence is applied. The NOESY difference spectrum obtained by subtracting a NOESY spectrum without presaturation of the I2-CH3 signal is shown in Fig. 8.23. Here, some more cross peaks are evident with respect to the 3D NOESY-NOESY experiment because secondary NOEs develop much more when the primary NOEs from the I2-CH3 signal evolve in a steady state experiment like the NOE-NOESY rather than in a transient-type experiment like the NOESY-NOESY. In Fig. 8.23, dipolar connectivity patterns are apparent among protons 

Other variants are NOE-COSY and NOE-TOCSY. However, these experiments are much less sensitive than NOE-NOESY. 

Sometimes it may be useful to suppress signals with long relaxation times in such a way as to detect cross peaks involving one fast relaxing signal under the envelope of diamagnetic signals. In this case a SuperWEFT sequence (180-r- 

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