AQA A Level Chemistry

Revision Notes

7.10.3 ¹³C NMR

¹³C NMR

  • Nuclear Magnetic Resonance (NMR) spectroscopy is used for analysing organic compounds
  • Atoms with odd mass numbers usually show signals on NMR
    • For example, isotopes of atoms
    • Many of the carbon atoms on organic molecules are carbon-12
    • A small quantity of organic molecules will contain the isotope carbon-13 atoms
    • These will show signals on a 13C NMR
  • In 13C NMR, the magnetic field strengths of carbon-13 atoms in organic compounds are measured and recorded on a spectrum
  • Just as in 1H NMR, all samples are measured against a reference compound – Tetramethylsilane (TMS)
  • On a 13C NMR spectrum, non-equivalent carbon atoms appear as peaks with different chemical shifts

Chemical shift values (relative to the TMS) for 13C NMR analysis table

Chemical shift values (relative to the TMS) for 13C NMR analysis table, downloadable AS & A Level Chemistry revision notes

 

Features of a 13C NMR spectrum

  • 13C NMR spectrum displays sharp single signals
    • There are no complicated spitting patterns like 1H NMR spectra
  • The height of each signal is not proportional to the number of carbon atoms present in a single molecular environment
  • Carbon atoms in different chemical environments will give resonances at different chemical shifts in a 13C spectrum
  • As with 1H NMR, tetramethylsilane is used as the standard reference point for 13C at 0 ppm

Identifying 13C molecular environments

  • On an organic molecule, the carbon-13 environments can be identified in a similar way to the proton environments in 1H NMR
  • For example propanone
    • There are 2 molecular environments
    • 2 signals will be present on its 13C NMR spectrum

There are 2 molecular environments in propanone

 

spectra, downloadable AS & A Level Chemistry revision notes

The 13C NMR of propanone showing 2 signals for the 2 molecular environments

Exam Tip

Counting the number of 13C resonances should be the first step in analysing a spectrum.

For example, it is possible to differentiate the three isomers of dihydroxybenzene quickly be considering the symmetry of the molecules and therefore the number of resonances expected in their spectra.

Worked Example

How many chemical environments and therefore number of peaks / resonances would be in a13C  spectra of 1,3-dihydroxybenzene?

Answer:

4 chemical environments and therefore four peaks / resonances on the spectra

 

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