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
• Lines = n + 1

  With n being the number of hydrogen on an adjacent carbon.
• Splitting

  So a doublet would mean that there is two line so using the lines = n+1 rule there would be 1 hydrogen on the adjacent carbon

  
• Splitting

  So for ethanol it would have 3 hydrogen environments and so 3 peaks in the 1HNMR spectrum.

  • For the first environment (CH3), its peak would be split into a triplet because it has two hydrogens on the adjacent carbon and using n+1 rule it would have 3 lines which is a triplets
  • For the second environment (CH2), its peak would be split into quartet because of the three hydrogens on the adjacent carbon
  • For the third environment (OH), its peak would be a singlet since there is 0 hydrogens on an adjacent carbon

  
• When analysing a sample in an NMR spectrometer it has to be dissolved in a solvent. The problem is you don’t want the solvent to produce any peaks of its own on the spectrum and interfere with the analysis.  For this reason the solvents used cannot contain any H atoms. Instead solvents using deuterium are used. 
• Deuterium is an isotope of hydrogen with a mass number of 2 (it has one proton and one neutron in the nucleus). For this reason it doesn’t create a peak in an NMR spectrum. Which means any solvent molecule where H has been replaced by D could be used. E.g. C6D6 (Deuterated benzene)
• State why Si(CH3)4 is used in 1H NMR spectrum

  Si(CH3)4 has an equal shape so it is not polar so it won't create a signal
• State why CCl4 is used in 1H NMR spectroscopy

  CCl4 doesn't have any hydrogen making it inert
• State why CDCl3 is used 1H NMR spectroscopy

  Because it used deuterium which is an isotope of hydrogen so it's inert and won't produce a signal
• Explain how the properties Si(CH3)4 , CCl4 and CDCl3 make them suitable for 1H NMR spectroscopy

  All of them are volatile making it easier to remove