Two-Dimensional Experiments that Show Exchange

Several NMR experiments can indicate the presence of chemical or conformational exchange. In some experiments, exchange produces cross peaks that are viewed as an annoyance. In other cases, the experiment may be carried out for the purpose of demonstrating the presence of exchange. 

The assignment of resonances to specific atoms in molecules can vary in difficulty from trivial to confounding. Some molecules lend themselves to resonance assignment readily vith the application of a fe v simple rules. For other molecules, ho vever, we make a series of preliminary assumptions or tentative assignments and then check our 

2-D cross peaks in the gHMBC and/or gCOSY to determine vhether we have a consistent set of assignments or (and this is more likely) a number of questionable, implausible, or far-fetched assignments that seriously call into question the validity of our tentative assignments. 

Resonances we assign with certainty are called entry points because they establish a beachhead or toehold by which we can progressively work across the molecule, accounting for all expected resonances. 

Different NMR experiments and even different types of information found in the same NMR data set (1-D or 2-D) provide sometimes conflicting implications regarding assignments. Entry points are typically those resonance assignments that are beyond reproach, those in which we place complete confidence. 

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