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NMR3. Symmetry in NMR

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    Butane shows two different peaks in the 13C NMR spectrum, below. Note that:

    the chemical shifts of these peaks are not very different from methane. The carbons in butane are in a similar environment to the one in methane.

    • NMR sy1.gif

      In the 13C NMR spectrum of pentane (below), you can see three different peaks, even though pentane just contains methyl carbons and methylene carbons like butane. As far as the NMR spectrometer is concerned, pentane contains three different kinds of carbon, in three different environments. That result comes from symmetry.

      NMR sy2.gif

      Figure NMR3.13C NMR spectrum of pentane.

      Source: SDBSWeb : (National Institute of Advanced Industrial Science and Technology of Japan, 15 August 2008)

      Symmetry is an important factor in spectroscopy. Nature says:

      • To learn about symmetry, take a model of pentane and do the following:
        • Wire Frame

          Ball & Stick

          Animation NMR1. A three-dimensional model of pentane. Grab the model with the mouse and rotate it so that you are convinced that the second and fourth carbons are symmetry-equivalent, but the third carbon is not.

          By the same process, you can see that the second and fourth carbons along the chain are also symmetry-equivalent. However, the middle carbon is not; it never switches places with the other carbons if you rotate the model. There are three different sets of inequivalent carbons; these three groups are not the same as each other according to symmetry.

    NMR3. Symmetry in NMR is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Chris Schaller.

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