Proton NMR Spectrometry

H-NMR spectrometry is a method used to figure out which organic compound we have in a sample. The sample is mixed with Si(CH3)4 and placed in a machine, where it is bombarded with magnetic fields. The data produced is used to determine which organic compound the sample contains.

How does
H-NMR work?
  1. Hydrogen atoms can behave like little compasses, which align along a magnetic field. 
  2. When a chemical bond (ie. C-H) is hit by a radio wave at just the right frequency, it flips its orientation from North to South and starts oscillating. This 'flipping' can be detected.
  3. The frequency needed to flip a hydrogen atom's orientation depends on what the atom is bonded to and the strength of the magnetic field. 
  4. We usually standardise the frequency and adjust the magnetic field slowly until a hydrogen atom starts oscillating.
  5. We then compare the magnetic field strength to a table with known values to determine what the atom is bonded to. 
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The output of an H-NMR looks like this.

We must pay attention to the chemical shift of each peak and the number of peaks.

The ppm value (chemical shift) is used to determine what the proton is bonded to by comparing it with a graph of known values.

The peaks tell us how many H atoms there are on surrounding carbon atoms. The number of H atoms is given by the equation: number of peaks - 1. 

  • The first peak is a triplet, meaning that it has 2 H atoms on adjacent C atoms. Its shift is between 0.9-1.2, meaning that it is a CH3 bonded to an alkyl group.
  • The second peak is a quartet, meaning that it has 3 H atoms on adjacent C atoms. Its shift is between 3.3-4.0, meaning that it is bonded to an O atom.
  • The third peak is a singlet, meaning that it is not a C atom. Its shift is between 1-5, meaning that it is an -OH group.
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