CIE A Level Chemistry (9701) 2019-2021

Revision Notes

8.1.7 Carbon-13 NMR Spectroscopy

Interpreting & Explaining Carbon-13 NMR Spectra

  • 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 as 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 for 1H molecular environments table

Analytical Techniques - 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 aren’t any complicated spitting pattern as seen with 1H NMR spectra
  • The height of each signal is not proportional to the number of carbon atoms present in a single molecular environment
  • CDCl3 is used as a solvent to dissolve samples for 13C NMR
    • On spectra, a single solvent peak appears at 80 ppm caused by 13C atoms in the CDCl
    • This can be ignored when interpreting 13C spectra

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

 

Analytical Techniques - Propanone Molecular Environments, downloadable AS & A Level Chemistry revision notes

There are 2 molecular environments in propanone

Analytical Techniques - Propanone 13C NMR Spectrum, downloadable AS & A Level Chemistry revision notes

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

Exam Tip

  • When a question is asking you to predict the number of molecular environments you would expect to see on a spectrum, it is a good idea to start by drawing the displayed formula
  • This then helps you to visualize the carbon atoms placement

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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