F324 Unit 4: NMR

NMR: nuclear magnetic resonance

• Requirements for NMR:
  • strong magnetic field applied using an electromagnet
  • low-energy radio frequency radiation
• Nuclear spin:
  • Protons and neutrons spin in opposite directions.
  • Nuclei with a magnetic spin are like tiny magnets and can either spin
    • With the field
    • Opposed to the field
  • Nuclei that oppose the field have a higher energy than those aligned
• Resonance:
  • Can promote a nuclei from low energy to high energy spin by excitation
    • This energy is supplied by low energy radio frequency waves
    • Excited nucleus will later drop back to low energy level by relaxation
  • Resonance is the cycle of excitation and relaxation.

Nuclear shielding:

• Magnetic field felt by a nucleus depends on:
  • The applied strong magnetic field
  • A weaker magnetic field generated from electrons surrounding nucleus and nearby atoms
• An atom's electrons shield the nucleus from some of the applied magnetic field - nuclear shielding. Extent of shielding depends on electron density.

Chemical shift:

• Place in the NMR spectrum at which the nucleus absorbs energy.
• Scale that compares frequency of an NMR absorption with frequency of reference peak at TMS = 0ppm
• TMS has 12 equivalent protons which give rise to a single NMR signal at 0ppm
• Add small amount of TMS when carrying out NMR to calibrate

Solvents for NMR are deuterated. They have an even number of nucleons and protons so no signal is produced.

Carbon-13 NMR spectroscopy

• Number of peaks = number of different carbon environments
• Chemical shifts = type of carbon environment
• Size of peak does not tell us anything

Proton NMR spectroscopy

• Number of…