NMR

• Types of NMR

  • 13C NMR
  • 1H (proton) NMR
• 13C NMR

  • There is only around 1% C13 in organic molecules but modern NMR machines are sensitive enough to give a full spectra for C13
  • The C13 spectra is a simpler spectrum than the H NMR
  • In a C13 NMR spectrum, there is one signal (peak) for each set of equivalent C atoms
• Equivalent carbon atoms


  Carbon atoms that are in the same chemical environment
• Equivalent hydrogen atoms


  Hydrogen atoms that are in the same chemical environment
• In an 1H NMR spectrum, there is one signal for each set of equivalent H atoms
• The intensity (integration value) of each signal is proportional to the number of equivalent H atoms it represents
• Ethanol

  • 3 groups of different hydrogen atoms: ratio 3:1:2
• CH3CH(CH3)CH2CH3

  • 3 sets of equivalent H's: ratio 3:2:3
• CH3C(CH3)2CH3

  • 3 sets of equivalent H's: ratio 3:2:9
• CH3CH2CH3


  • 3 sets of equivalent H's: ratio 3:1:2
• C6H6

  • 4 sets of equivalent H's: ratio 6:1:2:3
• Solvents

  • Samples are dissolved in inert solvents with no 1H atoms, e.g. CCl4, CDCl3
  • These solvents have no H atoms so will not give any peaks in the 1H NMR spectrum
  • The same solvents are used in 13C NMR and in this case there will be one peak due to the solvent that will appear on the spectrum. However, it is known where this peak is so it can be ignored.
• Calibration and shift

  • A small amount of TMS (tetramethylsilane) is added to the sample to calibrate the spectrum
  • The spectra are recorded on a scale known as the chemical shift (δ), which is how much the field has shifted away from the field for TMS
  • The δ is a measure in parts per million (ppm) is a relative scale of how far the frequency of the proton signal has shifted away from that for TMS
• TMS


  • It only gives one signal
  • Its signal is away from all the other H signals
  • Gives strong signal so only a small amount needed
  • It is non-toxic
  • It is inert
  • It has a low boiling point and so can be removed from sample easily
• CCl4

  Non-polar compound that is a good solvent for non-polar organic molecules
• CDCl3


  Polar covalent molecule that is a good solvent for polar organic molecules
• 1H NMR shift


  The δ depends on what other atoms/groups are near the H – more electronegative groups gives a greater shift
• C13 NMR shift


  • 160-185 ppm for C=O
  • 5-40 ppm for CH2
  • 20-40 ppm for CH
  • 50-90 ppm for C-O
• Hydrogen bonded to N or O


  Usually do not couple with other protons and appear as singlets on the 1H NMR spectra
• Spin-Spin coupling in 1H NMR

  • Splitting of peak = number of inequivalent H's on neighbouring C atoms + 1
  • Nuclei in identical chemical environments do not show coupling amongst themselves
• CH3CH2OH


  • The peak due to group a will be a triplet, the peak due to group b will be a quartet, the peak due to group c will be a singlet
• CH3CH2COOCH3

  • The peak due to group a will be a triplet, the peak due to group b will be a quartet, the peak due to group c will be a singlet
• C6H5NO2

  • The peak due to group a will be a singlet, the peak due to group b will be a quartet, the peak due to group c will be a triplet