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Multiplicity in Proton NMR

  • Page ID
    4188
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    Another type of additional data available from 1H NMR spectroscopy is called multiplicity or coupling. Coupling is useful because it reveals how many hydrogens are on the next carbon in the structure. That information helps to put an entire structure together piece by piece.

    In ethanol, CH3CH2OH, the methyl group is attached to a methylene group. The 1H spectrum of ethanol shows this relationship through the shape of the peaks. The peak near 3.5 ppm is the methylene group with an integral of 2H.

    • NMR mu8.gif

      The integral of 2H means that this group is a methylene, so it has two hydrogens. The carbon bearing these two hydrogens can have two other bonds.

      methylene.gif

      There could be two hydrogens on one neighbouring carbon and one on another. Otherwise, all three hydrogens could be on one neighbouring carbon.

      three neighbours.gif

      However, the shift of 3.5 ppm means that this carbon is attached to an oxygen. Mutliplicity usually only works with hydrogens on neighbouring carbons. If there is an oxygen on one side of the methylene, all three neighbouring hydrogens must be on a carbon on the other side.

      nmrethoxy.gif

      Alternatively, look at the spectrum the other way around. The peak at 1 ppm is the methyl group with an integral of 3H.

      • The neighbouring H could be on two different neighbouring carbons or both on the same one.

        2 neighbours.gif

        The number of lines in a peak is always one more than the number of hydrogens on the neighboring carbon. The triplet for the methyl peak means that there are two neighbors on the next carbon (3 - 1 = 2H); the quartet for the methylene peak indicates that there are three hydrogens on the next carbon (4 - 1 = 3H). Table NMR 1 summarizes coupling patterns that arise when protons have different numbers of neighbors.

        # of lines

         

        ratio of lines

         

        term for peak

         

        # of neighbors

         

        1

         

        -

         

        singlet

         

        0

         

        2

         

        1:1

         

        doublet

         

        1

         

        3

         

        1:2:1

         

        triplet

         

        2

         

        4

         

        1:3:3:1

         

        quartet

         

        3

         

        5

         

        1:4:6:4:1

         

        quintet

         

        4

         

        6

         

        1:5:10:10:5:1

         

        sextet

         

        5

         

        7

         

        1:6:15:20:15:6:1

         

        septet

         

        6

         

        8

         

        1:7:21:35:35:21:7:1

         

        octet

         

        7

         

        9

         

        1:8:28:56:70:56:28:8:1

         

        nonet

         

        8

         

        The third peak in the ethanol spectrum is usually a "broad singlet." This is the peak due to the OH. You would expect it to be a triplet because it is next to a methylene. Under very specific circumstances, it does appear that way. However, coupling is almost always lost on hydrogens bound to heteroatoms (OH and NH). The lack of communication between an OH or NH and its neighbours is related to rapid proton transfer, in which that proton can trade places with another OH or NH in solution. This exchange happens quite easily if there are even tiny traces of water in the sample.

        In summary, multiplicity or coupling is what we call the appearance of a group of symmetric peaks representing one hydrogen in NMR spectroscopy.


    Multiplicity in Proton NMR is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Chris Schaller.

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