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5.3: HMBC and HMQC Spectra

  • Page ID
    189777
  • Just as COSY spectra show which protons are coupled to each other, HMBC (and the related HMQC) give information about the relative relationships between protons and carbons in a structure.

    In an HMQC spectrum, a 13C spectrum is displayed on one axis and a 1H spectrum is displayed on the other axis. Cross-peaks show which proton is attached to which carbon. COSY spectra show 3-bond coupling (from H-C-C-H), whereas HMQC shows a 1-bond coupling (just C-H).

    Take a look at n-hexylbenzene. There are lots of similar positions in this structure, so it may be hard to tell peaks apart in either the proton or carbon NMR spectrum.

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    Indeed, the 1H spectrum has some unique peaks but also a couple of large multiplets.

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    Similarly, there are a couple of carbons that show up at the same place in the 13C NMR spectrum.

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    An HMQC experiment will help spread things out into two dimensions. Looking along the x axis, we can see that several peaks were clustered together near 7 ppm and near 1.3 ppm. Even looking along the y axis, we can see a couple of carbon peaks have separated out from each other around 33 ppm.

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    An HMBC spectrum looks very similar to an HMQC spectrum, except that it shows 2-bond, 3-bond or sometimes even 4-bond coupling (not H-C, but H-C-C or H-C-C-C or even H-C-C-C-C). Instead of seeing which carbon is directly attached to a hydrogen, we see which carbon is next to that one, so that we begin to see how the molecule connects.

    Exercise \(\PageIndex{1}\)

    Analyse the data to determine which of the two isomers (below) we are dealing with.

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    Answer

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    Exercise \(\PageIndex{2}\)

    Analyze the data to determine which of the two isomers (below) we are dealing with.

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    Answer

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    Exercise \(\PageIndex{3}\)

    Analyse the data to determine the structure of the compound.

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    Answer

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    Exercise \(\PageIndex{4}\)

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    Answer

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    Exercise \(\PageIndex{5}\)

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    Answer

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    Exercise \(\PageIndex{6}\)

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    Answer

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    Exercise \(\PageIndex{7}\)

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    Answer

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    Exercise \(\PageIndex{8}\)

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    Answer

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    *Sources:

    Selected IR spectra from SDBS (National Institute of Advanced Industrial Science and Technology, Japan, Spectral Database for Organic Compounds, http://sdbs.db.aist.go.jp/sdbs/cgi-bin/cre_index.cgi, accessed December, 2015).

    1H NMR, 13C NMR, HMBC and HMQC spectra simulated.