How to Solve Problems Involving Advanced 1-D Methods

The second spectrum was determined with the simultaneous irradiation of the methyl resonance at 1.9 ppm. We immediately see that the 1.9-ppm peak appears as a strongly negative peak. The only peak in the spectrum that appears as a positive peak is the vinyl proton peak at 5.5 ppm. The other vinyl peak at 6.1 ppm has nearly disappeared, as have most of the other peaks in the spectrum. The presence of a positive peak at 5.5 ppm confirms that this peak must come from proton Hz proton He is too far away from the methyl group to experience any dipolar relaxation effects. 

The above result could have been obtained by conducting the experiment in the opposite direction. Irradiation of the vinyl proton at 5.5 ppm would have caused the methyl peak at 1.9 ppm to be positive. The results, however, would not be very dramatic it is always more effective to irradiate the group with the larger number of equivalent hydrogens and observe the enhancement of the group with the smaller number of hydrogens rather than vice versa. 

This example is intended to illustrate how NOE difference spectroscopy can be used to solve complex structural problems. This technique is particularly well suited to the solution of problems involving the location of substituents around an aromatic ring and stereochemical differences in aUcenes or in bicyclic compounds. 

In this section, we will use some advanced 1-D NMR data to solve problems. As with any problan-solving exercise, having a systematic approach simplifies the task and helps ensure we do not overlook any information. We will begin with analysis of what sort of NMR experiments are likely to provide definitive information relevant to the question. It is then prudent to think about what one would observe for eaeh possible answer. Sometimes we may have to think about what would not be observed, but this is a potentially risky proposition. It is generally unwise to base conclusions on what might be called absent data. 

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