reversed NOESY/EXSY peaks
 

reversed NOESY/EXSY peaks

We have a case of a molecule that has two slowly exchanging conformations. In the NOESY experiment we see cross-relaxing cross-peaks (negative or the opposite sign to the diagonal) for interproton distances as well as exchanging cross-peaks (positive or the same sign as the diagonal). This is the classical expected behavior in such a sample. But...

In this molecule there is a pair of geminal protons for which the phenomenon is exactly reversed: The interproton cross-peak is POSITIVE and the exchange peaks are NEGATIVE. There is no doubt which peak is which (from the COSY, for instance).

I suspect a scrambling effect due to the combined mechanisms, but I hope to be reassured by a clear explanation or comments saying that this has been seen and described before.



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