NMR spectroscopy

• NMR stands for nuclear magnetic resonance

  • Nuclear relates to the nucleus of an atom
  • Magnetic - exhibiting or relating to magnetism, measured relative to magnetic north
  • Resonance - an oscillation or a subatomic particle that is a short lived excited state of a more stable particle
• The standard in a 13C spectra is tetramethylsilane (Si(CH3)4)
• NMR gives information about the position of 13C or 1H atoms
• 13C nuclei can act as a tiny magnet that lines itself up with the external magnetic field but 12C can't
• The most common isotope of carbon is 12C. This doesn’t have spin and so can’t be analysed by NMR spectroscopy. The isotope 13C has an abundance of about 1% and does have spin.
• TMS is used as a reference standard for chemical shifts. A few drops of TMS are added to the compound be analysed by the spectrometer
• Reasons why TMS is used as a standard are:

  • In generates one sharp peak.
  • It is unreactive, so won’t affect the sample being analysed.
  • It is highly volatile. This means it can be removed from the sample being tested after the analysis fairly easily.
• Number of Peaks = Number of Carbon Environments
• Calculate number of carbon environments in butane:

  There's 2 since the for the first and last carbon they are both bonded to 3 hydrogens and one carbon so they are in the same environment. For the middle carbons they are also attached to the same thing so they are in the same environment

  
• This is methylpropanoic acid. This has 3 carbon environments since there are two carbons in the same environment in peak 3 it is twice as larger

  
• 1H NMR (Proton NMR) is also used to analyse organic molecules as they usually contain Hydrogen atoms and is used more since hydrogen is easier to get compared to 13C
• Number of Peaks = Number of Proton/Hydrogen Environments
• This is butanone

  • It has 4 carbon enviroments
  • And 3 hydrogen enviroments

  
• This is ethanol. This would have 3 hydrogen environments for the 3 same hydrogens bonded to the carbon that would be 1 environment, the hydrogen on the second carbon is another environment and then the one on the oxygen

  
• The closer to an electronegative atom, the higher the chemical shift for that carbon.
• TMS is still used as a reference in proton NMR as it has a chemical shift of 0 ppm. It has just one proton environment and so gives one strong peak