• 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

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

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

Answer:

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