Interpreting & Explaining Carbon-13 NMR Spectroscopy (Cambridge (CIE) A Level Chemistry): Revision Note

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 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 TMS) for ¹³C NMR analysis table

   * represents a benzene ring

Features of a ¹³C NMR spectrum

• ¹³C NMR spectra display sharp single signals

  • There aren’t any complicated spitting patterns 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

Explaining 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

Identifying molecular environments in propanone

There are 2 molecular environments in propanone

• There are 2 molecular environments

  • Therefore, 2 signals will be present on its ¹³C NMR spectrum

• The carbonyl carbon will produce a ¹³C peak in the range of 190 - 220 ppm

• The 2 methyl groups will produce a ¹³C peak in the range of 30 - 65 ppm

Carbon-13 NMR spectrum for propanone

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