29.4 - proton NMR spectroscopy

• 4 important pieces of information provided by a proton NMR spectrum

  1. The number of different proton environments - from the number of peaks.

  2. The types of proton environments present - from the chemical shift.

  3. The relative numbers of each type of proton 0 from integration traces or ratio numbers of the relative peak areas.

  4. The number of non-equivalent protons adjacent to a given proton - from the spin-spin splitting pattern.
• What factors might move peaks on a proton NMR spectrum out of range?


  Solvent, concentration, and substituents.
• Equivalent protons


  If two or more protons are equivalent, they will absorb at same chemical shift, increasing size of peak.
• Non-equivalent protons


  Protons of different types have different chemical environments and are non-equivalent - they absorb at different chemical shifts.
• What does the ratio for relative areas under each peak tell us?


  The ratio of the number of protons responsible for each peak.
• Integration trace


  Measure of area under each peak. Shown as an extra line on the spectrum or a printed number of the relative peak areas.
• Spin-spin coupling


  A proton NMR peak can be split into sub-peaks or splitting patterns. These are caused by the proton's spin interacting with the spin states of nearby protons that are in different environments. This provides information about the number of protons bonded to the adjacent carbon atoms.
• n+1 rule


  For a proton with n protons attached to an adjacent carbon atom, the number of sub-peaks in a splitting pattern = n+1.

  
• Proton exchange


  A technique for identifying -OH and -NH protons:

  1. A proton NMR spectrum is run as normal.

  2. A small volume of deuterium oxide, D₂O, is added, the mixture is shaken and the spectrum is run.

  Deuterium does not absorb in this chemical shift range, and so the OH or NH peak disappears.
• Uses of two dueterated compounds in NMR spectroscopy


  1. CDCl₃ used as a solvent.

  2. D₂O used to identify NH or OH protons.