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Exchange Effects

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    79408
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    One final subtlety of coupling can be examined by considering the spectrum of a compound such as methanol (CH3OH). Using the rules for coupling we have established, predict the number of resonances and identify their multiplet structure.

    Methanol has two distinct hydrogen atoms, the three equivalent ones of the methyl group and the one of the hydroxyl group. These two different types of hydrogen atoms are three bonds away from each other so should couple. Therefore, the methyl hydrogen atoms should appear as a 1:1 doublet of area three and the hydroxyl hydrogen atom should appear as a 1:3:3:1 quartet of area one. In some situations, this is exactly how the NMR spectrum of methanol will appear.

    However, in other situations the spectrum of methanol appears as two singlets, one of area three and the other of area one. Propose a reason why singlets are observed instead of coupled multiplets.

    The important thing to recognize here is that the hydroxyl hydrogen atoms in methanol are exchangeable. The hydroxyl groups of methanol participate in hydrogen bonding and the hydroxyl hydrogen of one methanol associates with a lone pair on the oxygen atom of another methanol. Figure 25 illustrates a hydrogen bonded methanol dimer. When hydrogen bonding occurs, it is possible for the hydroxyl hydrogen atoms to exchange and leave with the other methoxy group as also shown in Figure 25. Note how the red and black hydroxyl hydrogen atoms end up with the other methoxy group in Figure 25. If the exchange rate is slow on the NMR time scale, coupling is observed because a methyl group is adjacent to a hydroxyl hydrogen atom whose magnetic field is aligned either “with” or “against” BAPPL during the entire time that the measurement is made. If the exchange rate is fast on the NMR time scale, during the course of the measurement, the methyl group is adjacent to many different hydrogen atoms because they are rapidly coming on and off. Considering the probabilities, half of these hydrogen atoms will be in the “with” orientation and half will be in the “against” orientation. The spectrum will be a time average with the result that the magnetic fields exactly cancel each other out such that no coupling is observed.

    Fig25.PNG
    Figure 25. Hydrogen bonding and exchange of hydroxyl hydrogen atoms between different methanol molecules.

    Another time-dependent aspect to consider about NMR spectra can be illustrated by considering the 1H NMR spectrum of N,N-dimethylacetamide (DMA) shown below.

    DMA.png

    What does the NMR spectrum of DMA look like?

    There are two different hydrogen atoms in DMA (methyl and acetaldehyde) that are four bonds removed from each other so there will be no coupling. Assuming rapid rotation of the carbonyl carbon-nitrogen bond, both methyl groups are equivalent. The NMR spectrum would therefore consist of two singlets, one for the methyl groups of area 6 and the other for the aldehyde hydrogen of area 1.

    DMA has the following contributing resonance form. What would the NMR spectrum of DMA look like in this resonance form?

    DMA_resonance.png

    In this form, there is no rotation of the carbonyl carbon-nitrogen double bond. Now the methyl groups are chemically inequivalent because one is cis and the other trans to the oxygen atom. The NMR spectrum would now consist of three singlets, two for the methyl groups, each of area 3, and the other for the aldehyde hydrogen of area 1.

    What ultimately happens is that the NMR spectra of DMA and other amides exhibit different behaviors at different temperatures. At high temperatures, the bond does undergo rapid rotation and the two methyl groups are equivalent. At low enough temperatures, the bond eventually undergoes slow rotation and the two methyl groups are inequivalent.


    This page titled Exchange Effects is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Thomas Wenzel via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.