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Chemistry LibreTexts

5.1: COSY Spectra

  • Page ID
    189775
  • In 2D NMR, correlation peaks are used to help establish the structure.

    Let's start with a simple compound, ethyl acetate.

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    The 1H NMR spectrum is not very complicated.

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    In homonuclear correlation spectroscopy (COSY), we can look for hydrogens that are coupled to each other. In ethyl acetate, it's pretty clear where they are. There is a quartet and a triplet; the hydrogens corresponding to those two peaks are probably beside each other in the structure.

    The COSY spectrum simply takes that 1H spectrum and spreads it out into two dimensions. Instead of being displayed as a row of peaks, the peaks are spread out into an array. In the following simulated COSY spectrum, the peaks are displayed along one axis. The same peaks are also displayed along the other axis. In the middle of the plot, the peaks are shown plotted against each other -- that is, there is a peak at 1.25 ppm on the y axis and along the x axis, so there is a data point on the plot at (1.25, 1.25). Other such peaks appear along the diagonal stretching from the upper right to the lower left through the middle of the plot. There are three peaks appearing along that diagonal line; these three peaks basically show us what we already saw in the regular 1H NMR spectrum.

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    The interesting peaks are the ones that do not appear along the diagonal. Those peaks indicate which hydrogens are coupled to which other hydrogens. The hydrogens at 1.25 ppm are coupled to the ones at 4.2 ppm, and that gives a "cross-peak" at (1.25, 4.20). There is also a cross-peak at (4.20, 1.25), because that relationship goes both ways.

    What does it mean to be coupled? It means that magnetic information is transmitted between the atoms. How can we tell? Essentially, we can send a pulse of electromagnetic radiation into one set of hydrogens and look for a response somewhere else. Of course, if we send a pulse of radio waves at a frequency that will be absorbed by a particular hydrogen, we will see a response in that hydrogen itself. That's why we see the peaks on the diagonal. However, we also see responses from other hydrogens that are magnetically linked to the original one.

    A second example is shown below. This time, the compound is propanamine. In propanamine, there would again be a diagonal set of peaks, shown in black, corresponding to the peaks in the regular proton spectrum. In addition, two different sets of protons are coupled. The methyl group is coupled to the methylene in the middle of the molecule; that cross-peak is shown in blue. The methylene in the middle is also coupled to the methylene next to the nitrogen; that cross-peak is shown in red.

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    Exercise \(\PageIndex{1}\)

    Provide analyses of the following spectra and propose a structure for the compound.

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    Answer

    ethyl butanoate

    Exercise \(\PageIndex{2}\)

    The following COSY spectrum is for an isomer of dinitrobenzene. Which isomer is it?

    clipboard_e028527eef1d69af067d28f8ef2bbaaa7.png

    Answer

    clipboard_e5bce9169fe35edfe2e742607479c4ac2.png

    Exercise \(\PageIndex{3}\)

    Analyse the data in the following spectra and propose a structure for the compound.

    clipboard_e1f095c76a1b9313857441870e1f7dfef.png

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    Answer

    clipboard_e0a55e4f8927b4a344906679baf3724ce.png

    Exercise \(\PageIndex{4}\)

    Analyse the data in the following spectra and propose a structure for the compound.

    clipboard_ecf769630fda25403c337e4ef678bda3f.png

    clipboard_e01e85f40f4cff38b921d491c3e8ebe24.png

    Answer

    clipboard_e5c8eb5872217c6a37dc54a46a1cece06.png

    Exercise \(\PageIndex{5}\)

    Analyse the data in the following spectra and propose a structure for the compound.

    clipboard_e74c5b851d8aff66eee564d274e9f57e3.png

    clipboard_edd5b59c348ef873aeabba4cf24566e94.png

    Answer

    clipboard_e131a2c6553392448a37c52cefad9ca39.png

    Exercise \(\PageIndex{6}\)

    Analyse the data in the following spectra and propose a structure for the compound.

    clipboard_e134aafe5a05840c90939e1b97b654449.png

    clipboard_e6e586fdb5a73dbb7269cdb7b4230d089.png

    Answer

    clipboard_e91cb0902ded3fe6d6016a6e90904cc73.png

    Exercise \(\PageIndex{7}\)

    Analyse the data in the following spectra and propose a structure for the compound.

    clipboard_e3e7fda9be2701e913d57a1915632cc9c.png

    clipboard_ef041a50c9b8499b8788584fa1864722d.png

    Answer

    clipboard_e4f185c633376c33f11be9fb4b624d8e8.png

    Exercise \(\PageIndex{8}\)

    Analyse the data in the following spectra and propose a structure for the compound.

    clipboard_efd5d1b0ce9e83c1f966df19b8d9dcd61.png

    Answer

    clipboard_e149839961c6b6f8ece1e8aa92a9dff6d.png

    Exercise \(\PageIndex{9}\)

    Analyse the data in the following spectra and propose a structure for the compound.

    clipboard_e21d03ebc56b0e3947ab860f6b58d138a.png

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    Answer

    clipboard_ed732c46ac92279d066de4afabb32c67f.png

    Exercise \(\PageIndex{10}\)

    Analyse the data in the following spectra and propose a structure for the compound.

    clipboard_ef712eda702b93a3aa6d05389ff374149.png

    clipboard_ee399dee3be9e99349471e251dca7084d.png

    Answer

    clipboard_e159a77b7ed8e4aed8fae4e198e327ff4.png

    Exercise \(\PageIndex{11}\)

    Analyse the data in the following spectra and propose a structure for the compound.

    clipboard_ef7f94e6398b2f2d1eceb1c7d3ad02384.png

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    Answer

    clipboard_eb49fb4ba8680407a9c48b7bf17832431.png

    Exercise \(\PageIndex{12}\)

    Analyse the data in the following spectra and propose a structure for the compound.

    clipboard_e4209063974c84d18fc8ff7f1a81f6992.png

    clipboard_eb21b70b85028de9b45c468bd61c78412.png

    Answer

    clipboard_eb8ed32490193c2bd15e66c52f88928f7.png

    * Sources:

    Selected IR and 13C NMR 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 and COSY spectra simulated.