Structure determination

• NMR = Nuclear Magnetic Resonance


  • molecules/atoms have spin property
  • when magnetic field applied -> all spin in same direction
  • increasing intensity of field causes spin to keep flipping
  • different molecules flip at different intensities
  • suitable isotopes for NMR:
  • 1-H
  • 13-C
  • 31-P
  • 19-F
• Isotopes

  • 1-H NMR:
  • gives lots of structural information
  • quick -> 1 minute
  • 13-C NMR:
  • slower -> 5 minutes
• The Chemical Shift

  • chemical shift = position of peak in spectra
  • affected by environment -> shielding of nuclei
  • less e- / more electronegative atoms neighbouring H atom -> less shielding of nuclei -> higher magnetic field exposed to nuclei -> harder to flip -> higher ppm
  • deshielding effect = electronegative atoms withdraw e-
  • shielding effect = electron donating groups shield & lower ppm
• TMS - tetramethylsilane

  • Si(CH3)4
  • added to calibrate spectrum (set to 0)
  • only 1 strong signal, non-toxic & inert
• 1-H NMR

  • have to use different solvents - can't have H in solvent
  • 2-H = Deuterium used as a replacement for 1-H
  • spin-spin coupling = when the applied magnetic field is affected by the magnetic field of the H atoms on neighbouring carbon atoms
  • causes peak to split in proportions following Pascal's Triangle
  • n+1 rule
  • integration trace = difference in the size of peaks determined by the proportion of Hs in different environments