8.1.4 Interpreting & Explaining Carbon-13 NMR Spectroscopy

• 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 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

There are 2 molecular environments in propanone

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