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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.

asked Oct 12 '15 at 02:03

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foxx
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updated Oct 12 '15 at 05:12


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Hi foxx,

A recent article by Kuprov et al may provide some hints (I can't say that this is the issue in your case, but this is probably worth a look):

http://onlinelibrary.wiley.com/doi/10.1002/anie.201410271/pdf

Anomalous Nuclear Overhauser Effects in Carbon-Substituted Aziridines: Scalar Cross-Relaxation of the First Kind. Ilya Kuprov, David M. Hodgson, Johannes Kloesges, Christopher I. Pearson, Barbara Odell, and Timothy D. W. Claridge, Angew. Chem. Int. Ed. 2015, 54, 3697-3701.

Pete

link

answered Nov 23 '15 at 11:26

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Pete Gierth
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Asked: Oct 12 '15 at 02:03

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Last updated: Nov 23 '15 at 11:26

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