Carbon-13 NMR (Edexcel A Level Chemistry): Revision Note

Using Carbon-13 Data

• 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 splitting 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

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

• The small peak at about 77 ppm comes from the solvent, CDCl~3~, so it is not counted as a propanone signal