29.3 - carbon-13 NMR spectroscopy

• Two important pieces of information about a molecule provided by a carbon-13 NMR spectrum


  1. The number of different carbon environments (from the number of peaks).

  2. The types of carbon environment present (from the chemical shift).
• 4 main types of carbon atom that absorb over different chemical shift ranges

  - C=O

  - C=C

  - C-X (X = electronegative atom)

  - C-C
• Why may chemical shifts be out of the given ranges?


  Due to the solvent, concentration, and substituents.
• What does the chemical environment of a carbon atom determined by?


  The position of the atom within the molecule.
• What happens to the chemical shifts of a molecule if the carbon atoms are bonded to different atoms or groups of atoms?


  The carbon atoms will have different chemical environments and will absorb different chemical shifts.

  
• What happens to the chemical shifts of a molecule if two carbon atoms are positioned symmetrically?


  The two carbon atoms are equivalent and have the same chemical environment. They will then absorb radiation at the same chemical shift and contribute to the same peak.