NMR Spectroscopy

• • In Nuclear magnetic resonance (NMR) spectroscopy- a sample of a compound is placed in a strong magnetic field and exposed to a range of different frequencies of low-energy radio waves.
  • The nuclei of certain atoms within the molecule absorb energy from the radio waves. The amount of energy that a nucleus absorbs at each frequency will depend on the environment that it's in.
  • The pattern of these absorptions gives you information about the positions of certain atoms within the molecule, and about how many atoms of that type the molecule contains.
• The types of NMR Spectroscopy

  • The two types of NMR spectroscopy are carbon-13 NMR and high resolution proton NMR.
  • Carbon-13 NMR gives you information about the number and type of carbon environments that are in a molecule.
  • Carbon 12 isn’t detected in the sample because it has an even number protons, carbon 13 can be detected.
  • High resolution proton NMR gives you information about the number of hydrogen atoms that are in a molecule, and the environments that they’re in.
• Nuclear environments

  • A nucleus is partly shielded from the effects of an external magnetic field by its surrounding electrons. Any other atoms and groups of atoms that are around a nucleus will also affect the amount of electron shielding.
  • So the nuclei in a molecule feel different magnetic fields depending on their environments.
  • The more electron shielding the less affected they are by the magnetic field, this results in a lower chemical shift on the spectrum.

  
• Nuclear environments (2)

  • An atom's environment depends on all the groups that it's connected to, going right along the molecule -not just the atoms it's directly bonded to.
  • To be in the same environment, two atoms must be joined to exactly the same things.

  
• Chemical shift

  • Chemical shift (delta)- the differences in the energy absorbed by nuclei in different environments relative to a standard substance.
  • The standard substance is tetramethylsilane (TMS).
  • This molecule has 12 hydrogen atoms in identical environments and 4 carbon atoms in identical environments. This means that, in both proton NMR and C-13 NMR, it will produce a single absorption peak.

  
• Chemical shift (2)

  • Chemical shift is measured in parts per million (ppm) relative to TMS.
  • So the single peak produced by TMS is given a chemical shift value of 0.
  • TMS is added to the test compound for calibration purposes.
  • It's inert and has very little polarity, so lots of shielding resulting in a low chemical shift.
• Chemical shift (3)

  • NMR spectra are recorded with the molecule that is being analysed in solution.
  • But if you used an ordinary solvent, like water or ethanol, the hydrogen nuclei in the solvent would add unwanted peaks to the proton NMR spectrum.
• Chemical shift (4)

  • To overcome this, the hydrogen nuclei in the solvent are replaced with deuterium an isotope of hydrogen with one proton and one neutron.
  • Deuterium nuclei don't absorb the radio wave energy, so they don't add peaks to the spectrum.
  • The sample has to be dissolved in a solvent called (e.g. CDCI3) it gives an easily recognisable signal which the spectrometer removes from the spectrum.