Proton NMR

• • Each peak on a proton NMR spectrum is due to one or more hydrogen nuclei (protons) in a particular environment.
  • The relative area under each peak tells you the relative number of H atoms in each environment.
  • The splitting number is determined by how many hydrogens are on the next carbon. (Splitting-1 = no. of hydrogens on next carbon)

  
• • You can predict the number of peaks on a proton NMR spectrum, and the ratio of the areas under each peak, by looking at the structure of a molecule.

  
• Integration traces


  • Proton NMR spectra can get quite cramped. Sometimes it's not easy to see the ratio of the areas - so an integration trace is often shown.
  • The height increases shown on the integration trace are proportional to the areas of the peaks.

  
• Splitting patterns

  • The peaks on a proton NMR spectrum may be split into smaller peaks (this is called spin-spin splitting).
  • These split peaks are called multiplets. The splitting is caused by the influence of non-equivalent hydrogen atoms that are bonded to neighbouring carbons - these are carbons one along in the carbon chain from the carbon the hydrogen's attached to.
  • Peaks always split into the number of non-equivalent hydrogens on the neighbouring carbons, plus one.
  • It's called the n + 1 rule.

  
• Identifying OH and NH protons

  • The chemical shift due to protons (H atoms) attached to oxygen (OH) or nitrogen (NH) is very variable, they make a broad peak that isn't usually split (it's a singlet).
  • A trick to identify OH and NH protons includes running two spectra of the molecule — one with a little deuterium oxide, D2O, added.
  • If an OH or NH proton is present it'll swap with deuterium to become an OD or ND group.
  • The peak that was caused by that group will disappear. (This is because deuterium doesn't absorb the radio wave energy).

  
• Identifying OH and NH protons (2)

  
• Things to look out for when predicting structures from proton NMR spectra:

  • The number of peaks tells you how many different hydrogen environments there are in your compound.
  • You can use the chemical shift of each peak to work out what type of environment the hydrogen is in.
  • The ratio of the peak areas tells you about the relative number of hydrogens in each environment.
  • The splitting pattern of each peak tells you the number of hydrogens on the adjacent carbon. You can use the n+1 rule to work this out.

  
• Predicting structures from proton NMR spectra (2):

  
• Predicting proton NMR spectra

  
• Combining Spectra

  
• Combining Spectra (2)

  
• Combining Spectra (3)

  
• Combining Spectra (4)

  
• Combining Spectra (5)