Suggestions for spectral assignment

13C or 15N resonances that escaped detection through inverse detection technique, direct detection of 13C or 15N can be used [52]. 

At the end of this book it may be useful to summarize a possible strategy for spectral assignments. Some hints are so simple that they have never been mentioned previously, whereas other hints are implicit in the various parts of the book. 

The first criterion really related to the content of this book is the analysis of T and 72. As the dominant contribution to nuclear relaxation is dipolar in nature, Tfl and linewidths will decrease as we move farther from the paramagnetic center. Even the contact contribution to relaxation often decreases with the number of chemical bonds from the paramagnetic center. A caveat, however, should be given. Spin density transfer causes ligand-centered relaxation. Significant spin density on ax orbital of an sp2 carbon may relax an attached proton more than the paramagnetic center itself, owing to the different distances and to the sixth power dependence on distance. 

With relaxation data at hand, and with some experience, a tentative assignment can be proposed. This is how NMR spectroscopists of paramagnetic molecules operated from the 1960s to the 1980s. The first order J splitting is only rarely observed in paramagnetic molecules, and therefore is not a useful tool for the assignment. 

At this point NOE and NOESY experiments are needed. In this way we learn which proton is close to which other proton, and the picture becomes sharper. Sometimes, when the correlation times governing cross relaxation are unfavorable, ROE or ROESY experiments can be a valid alternative. Both NOE and ROE types of experiment also provide information on the presence of chemical equilibria when the interconversion rate is of the order of T. It is also clear that NOE and ROE types of experiment may not always provide a unique picture, because they ignore chemical bonds. COSY and TOCSY experiments provide the information on which proton is chemically bound to which other proton. 

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