Introduction to Structure Determination  
NMR: Coupling  
Introduction   Infrared   Ultraviolet   Mass Spectrometry   NMR   Crystallography   Combined  
 
  • Introduction
  • Chemical Shifts
  • Equivalent Nuclei
  • Coupling
  • Integration
  • The Spectrum
  • Examples
  • Self Assessment Problems
    •
    Characteristic Chemical Shifts
    One of the main benefits of NMR over IR and UV spectroscopy and mass spectrometry is that neighbouring nuclei interact ("couple") with each other. The chemical shifts give information on what functional groups are present and the coupling gives information on what groups are bonded to each other. NMR not only provides the pieces of the jigsaw but also how to fit them together.

    The chemical shift of a nucleus depends sensitively on how much of the applied magnetic field is shielded by the electrons. Nearby nuclei with spin are also a source of magnetism. The additional magnetism from nearby nuclei will affect the magnetic field that is experienced by a nucleus.

    The picture to the left shows ethanal. There are two 1H environments. The 3 H on the CH3 group are equivalent and are next to a carbonyl group so are predicted to have a chemical shift in the range 2.0 - 2.7. The H on the C=O group is on a carbonyl group so is predicted to have a chemical shift in the range 9.4 - 10.4.    		   Coupling to make an NMR doublet
    In addition to the applied magnetic field, the 3 H on the CH3 group experience an additional magnetic field due to the H on the C=O group. If the spin on H is aligned with the applied field ('up'), it will add to the magnetic field experienced by the 3 H atoms increasing their chemical shift. If the spin on the H is aligned against the applied field ('down'), it will decrease the magnetic field experienced by the 3 H nuclei decreasing their chemical shift. This is shown in the picture on the right.

    In a large sample, there is a 50 : 50 chance of each occuring and so the signal due to the 3 H on the CH3 group is split in two: a 'doublet'. Note that the signal due to the 3H appears as a doublet because it is next to 1 H. The number of lines gives information on the number of neighbours.
    Coupling to make an NMR quartet   In the same way, the magnetic field experienced by the H on the C=O group depends on the spins of the 3 H on the CH3 group. Each of these can individually be either aligned with ('up') or against ('down') the applied magnetic field. Taken together as a set, there are 4 possibilities as pictured on the left:
    • all 3 spins up (1 way of doing this)
    • 2 spins up and 1 spin down (3 ways of doing this)
    • 1 spin up and 2 spins down (3 ways of doing this)
    • all 3 spins down (1 way of doing this).
    Each of the 4 possibilities changes the magnetic field experienced by the H atom differently. The signal is split into 4: a 'quartet'. Because the probability of achieving 2 up / 1 down or 1 up / down is 3 times that of achieving all 3 up or all 3 down, the 4 peaks in the quartet have relative sizes 1 : 3 : 3 : 1.

    Note again that the signal due to the 3 H appears as a quartet because it is next to 3 H. The number of lines gives information on the number of neighbours.

    All of this may remind you of the odds when flipping coins. Both are governed by the same thing: probability:
    1. If 1 coin is tossed, there are 2 outcomes and getting heads or tails is equally likely. If there is coupling to 1 nuclei, a doublet is produced with peak heights in the ratio 1 : 1.
    2. If 2 coins are tossed, there are 3 outcomes (2 heads, 1 head & 1 tail and 2 heads) and the middle one is twice as likely as it can be achieved in 2 ways (head flilowed by tail or tail flilowed by head). If there is coupling to 2 nuclei, a triplet is produced with peak heights in the ratio 1 : 2 : 1.
    3. If 3 coins are tossed, there are 4 outcomes (3 heads, 2 heads & 1 tail, 1 head & 2 tails and 3 tails) with likelihoods in the ratio 1 : 3 : 3 : 1. If there is coupling to 3 nuclei, a quartet is produced with peak heights in the ration 1 : 3 : 3 : 1.
    All of this can be generalized:
    • NMR signals are split due to coupling and the number of lines (and the pattern) gives information on the number of H atoms on the neighbouring atoms
    • A signal is split into "n + 1" lines. The number of lines is called the 'multiplicity'.

      The signal from a 1H nucleus with "n" chemically equivalent neighbouring 1H nuclei will be split into n + 1 peaks.

    • The peak heights of these lines are given by Pascal's triangle:
    Neighbours Peaks Multiplicity Pattern
    0 1 singlet 1
    1 2 doublet 1 : 1
    2 3 triplet 1 : 2 : 1
    3 4 quartet 1 : 3 : 3 : 1
    4 5 quintet 1 : 4 : 6 : 4 : 1
    5 6 sextet 1 : 5 : 10 : 10: 5 : 1
    6 7 septet 1 : 6 : 15 : 20 : 15: 6 : 1


    Coupling between equivalent nuclei

    No coupling is observed between nuclei which are equivalent. For example, the 3 H on the CH3 in ethanol are all equivalent and no coupling between these nuclei is observed. Only the coupling of the H nuclei to the H nucleus (and vice versa) is observed.

    Effects of 13C and 13C NMR

    In a 1H NMR spectrum, coupling to the 13C nuclei is not usually observed. Because of the low abundance of 13C, the chance of a 1H nucleus being next to a 13C nucleus is very low and so coupling only occurs in a small portion of the sample.

    IN contrast, in a 13C NMR spectrum, every 13C nucleus will be in molecules containing 1H nuclei so coupling does occur. However, 13C NMR spectrometers are usually set up to remove the coupling. As a result:
    no coupling is seen in the 13C NMR spectra.
     
     © Prof Adam Bridgeman, School of Chemistry, The University of Sydney, 2024