NMR

• What does NMR stand for

  Nuclear Magnetic Resonance
• What is the principle of NMR

  • You can find the structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them
  • If radio waves of the right frequency are absorbed the nuclei flips.
  • This energy can be monitored and recorded
• How would you carry out NMR

  • Dissolve the LIQUID sample in a suitable solvent
  • Put it into a tube with a small drop of TMS and put the tube into an NMR machine
  • The sample is spun to even out any imperfections in the magnetic field and the spectrometer is zeroed against TMS
  • Radiation with different frequencies but a constant magnetic field is applied to the sample, an absorbances are detected
• Give one use of NMR

  MRI scans
• What kind of nuclei does NMR work with

  Those with an UNEVEN number of NUCLEONS meaning they spin

  1H

  13C
• What percentage of carbon atoms are 13C

  • 1%
  • But modern instruments are sensitive enough to detect this
• What defines the resonant frequency of 13C atom

  • The chemical environment that it is in; the amount of electron shielding it has
• What graph is produced by NMR spectroscopy

  Energy absorbed against chemical shift
• What is chemical shift

  • The resonant frequency of the nuclei, compared to that of a 1H atom in TMS
  • Its measured in Parts Per million (ppm)
• What is range of chemical shift for 13C NMR

  0-200ppm
• What means 13C atoms show a different chemical shift value

  • Having different chemical environements (but equivalent atoms show the same peak)
• What kind of environment leads to a greater chemical shift

  A C atom next to more electronegative atom has a greater chemical shift
• What does the Number of SIGNALS mean in C13 NMR

  One signal = for each carbon environment (each set of inequivalent 13C)
• What does the Chemical SHIFT mean in the C13 NMR

  Greater chemical shift from atoms CLOSER to electronegative atoms or C=C
• What does the area under the PEAK mean for 13C NMR

  NO MEANING
• What does the splitting mean for 13C NMR

  there is NO splitting for 13C NMR
• Why is it easier to get a spectrum of 1H NMR than 13C NMR

  • Most H atoms are 1H -> It is much more abundant than 13C
  • This means almost all H atoms have spin so they show up
• What is the range of chemical shift for 1H NMR

  0-10ppm
• What leads to a LOWER chemical shift value for H NMR

  • 1H with more electrons around them
  • Such as from further electronegative groups/atoms
• On a low resolution spectrum, what peaks would you expect to see for H NMR 


  ONE PEAK for each set of inequivalent H atoms

  (each chemical environment shows 1 peak)
• What does the area UNDER the peak represent for H NMR

  The area under the peak is PROPORTIONAL to the number of 1H atoms represented by the peak
• What is the integration trace

  • A stepped line that makes it easier to measure the area under the curve
  • (Height of line = area under the peak)
• What is TMS

  Tetramethylsilane

  
• What state is TMS at room temperature

  liquid
• Why is TMS used

  • Can be added to a sample to calibrate the NMR equipment
  • It provides a peak at exactly chemical shift = 0ppm
  • It is a reference point against which all are measured
• What are the advantages of using TMS

  • Inert
  • NON-toxic
  • Easy to remove from the sample
• When does splitting/spin-spin coupling occur

  Neighbouring hydrogen atoms (3 or fewer bonds away, or on the adjacent carbon) affect the magnetic field of 1H atoms and causes their peaks to split
• What is the n+1 rule

  If there are n inequivalent 1H atoms on the neighbouring carbon then the peak will split into (n+1) smaller peaks
• Draw the splitting pattern for 0 inequivalent H atoms 3 bonds or less away

  singlet

  
• Draw the splitting pattern for 1 coupled proton 3 bonds or less away

  Doublet

  
• Draw the splitting pattern for 2 coupled protons 3 bonds or less away

  triplet

  
• Draw the splitting pattern for three coupled protons 3 bonds or less away

  quartet

  
• Why must solvents used for 1H NMR NOT contain any hydrogen atoms

  • Signals from the solvent would swamp signals from the sample
  • Because there is much MORE solvent sample
• Which solvents are used in NMR

  • Deuterated solvents -> CDCl3, D2O, C6D6
  • CCl4 -> tetrachloromethane
• What does the number of signal mean in 1H NMR

  One main signal for EACH set of inequivalent 1H atoms ( for each hydrogen environment)
• What does the chemical shift mean in 1H NMR

  A LARGER chemical shift is present for 1H atoms CLOSER to the electronegative atoms OR C=C
• What does splitting mean in 1H NMR

  The number of SMALLER peaks = 1 + the number of inequivalent hydrogen atoms 3 bonds away
• What does the area under the peak mean in 1H NMR

  It is proportional to the number of atoms represented by that peak
• Why does the peak from O-H bonds DISAPPEAR if D2O is used as a SOLVENT

  • O-D is formed in preference to O-H due to labile protons that move/swap from one molecule to another