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6.8: Principles of ¹³C NMR Spectroscopy

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    227586
  • Most of what we have learned about 1H-NMR spectroscopy also applies to 13C-NMR, although there are several important differences.

    Number of signals in 13C-NMR spectroscopy

    In analogy to 1H NMR, 13C NMR can be used to obtain information about the C-C skeleton in an organic compound. The number of signals in a 13C NMR spectrum is related to the number of nonequivalent carbons in the chemical structure of the compound. For example, the 13C NMR spectrum of ethyl acetate shows four signals, one for each of the nonequivalent carbons.

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    Because of the low natural abundance of 13C nuclei, it is very unlikely to find two 13C atoms near each other in the same molecule, and thus we do not see spin-spin coupling between neighboring carbons in a 13C-NMR spectrum. There is, however, heteronuclear coupling between 13C carbons and the hydrogens to which they are bound. Carbon-proton coupling constants are very large, on the order of 100 – 250 Hz. For clarity, chemists generally use a technique called broadband decoupling, which essentially 'turns off' C-H coupling, resulting in a spectrum in which all carbon signals are singlets. Above is the proton-decoupled13C-NMR spectrum of ethyl acetate, showing the expected four signals, one singlet for each of the carbons.

     

    Example

    How many sets of non-equivalent carbons are there in:

    1. toluene
    2. 2-pentanone
    3. para-xylene

    Solution

     

    13C NMR Spectral window

    One of the greatest advantages of 13C-NMR compared to 1H-NMR is the breadth of the spectrum - recall that carbons resonate from 0-220 ppm relative to the TMS standard, as opposed to only 0-12 ppm for protons. Because of this, 13C signals rarely overlap, and we can almost always distinguish separate peaks for each carbon, even in a relatively large compound containing carbons in very similar environments. In the proton spectrum of 1-heptanol, for example, only the signals for the alcohol proton (Ha) and the two protons on the adjacent carbon (Hb) are easily analyzed. The other proton signals overlap, making analysis difficult.

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    In the 13C spectrum of the same molecule, however, we can easily distinguish each carbon signal, and we know from this data that our sample has seven non-equivalent carbons. (Notice also that, as we would expect, the chemical shifts of the carbons get progressively smaller as they get farther away from the deshielding oxygen.)

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    This property of 13C-NMR makes it very helpful in the elucidation of larger, more complex structures.

    13C NMR Chemical Shifts for functional groups

    The Carbon NMR is used for determining functional groups using characteristic shift values. 13C chemical shifts are greatly affected by electronegative effects. If a H atom in an alkane is replaced by substituent X, electronegative atoms (O, N, halogen), 13C signals for nearby carbons shift downfield (left; increase in ppm) with the effect diminishing with distance from the electron withdrawing group. Figure 13.11.1 shows typical 13C chemical shift regions of the major chemical class.

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    Figure 13.11.1: 13C Chemical shift range for organic compound

     

    13C NMR Chemical Shifts Intensity

    Unlike 1H-NMR signals, the area under a 13C-NMR signal cannot be used to determine the number of carbons to which it corresponds. This is because the signals for some types of carbons are inherently weaker than for other types – peaks corresponding to carbonyl carbons, for example, are much smaller than those for methyl or methylene (CH2) peaks. Peak integration is generally not useful in 13C-NMR spectroscopy, and therefore, the most useful information provided by the 13C-NMR spectrum is the number and chemical shift of the signals, but no integration or multiplicity of signals.

     

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