C-13 NMR

• Interpreting C-13 NMR spectra

  1. Count the number of peaks in the spectrum (excluding the TMS peak)- this number is equal to the number of carbon environments in the molecule.
  2. Use a diagram of chemical shift data to work out what kind of carbon environment is causing each peak.
  3. Use this information to figure out the structure of the molecule.

  
• Example 1

  
• C-13 NMR spectra of cyclic molecules (example)

  • The number of peaks on the C-13 spectrum of a cyclic compound depends on the symmetry of the molecule.

  
• C-13 NMR spectra of cyclic molecules (example) (2)

  1. The spectrum has four peaks, so it must have four carbon environments.
  2. All four peaks are between 120 ppm and 140 ppm.
  3. Looking at the chemical shift table these can only be due to alkene groups or carbons in a benzene ring. Since the question tells you that the molecule is aromatic, these carbons must be in a benzene ring.
  4. There are only three aromatic molecules with the formula C6H4CI2 — they're all isomers of dichlorobenzene.
  5. So the spectrum must have been produced by 1,3-dichlorobenzene.

  
• Predicting C-13 NMR spectra

  1. Identify all the different carbon environments present in the molecule.
  2. Look up the chemical shift values of the different environments in a chemical shift diagram.