C-13 NMR

• A carbon-13 NMR spectrum provides two important pieces of information about a molecule:

  • the number of different carbon environments - from the number of peaks
  • the types of carbon environment present - from the chemical shift
• The chemical shift is referenced against TMS at 0 ppm
• The chemical shift range of about 220 ppm is sufficiently

  wide to separate carbon atoms that have only slightly different environments
• Four main types of carbon atom that absorb over different chemical shift ranges:

  1. Carbon bonded to carbon
  2. Carbon bonded to an electronegative atom e.g. O, N, Cl, Br
  3. Carbon as part of C=C or aromatic ring
  4. Carbon as part of C=O
• The chemical shifts may also be outside of these ranges, depending on the solvent, concentration, and substituents
• The chemical environment of a carbon atom is determined by the position of the atom within the molecule
• If two carbon atoms are positioned symmetrically within a molecule, then they are equivalent and have the same chemical environment. They will then absorb radiation at the same chemical shift and contribute to the same peak.
• Carbon atoms that are bonded to different atoms or groups of atoms have different environments and will absorb at different chemical shifts.
• If a carbon falls into the same broad C—C environment as another carbon, but one is nearer to an oxygen atom connected to another carbon it is likely to be shifted more
• Interpreting C-13 spectra

  1. Draw out the isomers of the given molecular formula and identify the number of chemical environments for the carbon atoms which equals the expected number of peaks
  2. Match the structures to the spectra from the number of peaks
  3. Assign the peaks using the chemical shifts (ppm)
• The number of peaks is the same as the number of environments
• Predicting a C-13 NMR spectra

  1. Draw out the structure
  2. Identify the number of chemical environments = number of peaks on spectra
  3. Using a data sheet predict the chemical shifts