What is Nuclear Magnetic Resonance (NMR)?

 

This is part of the HSC Chemistry course under Module 8 Section 2: Analysis of Organic Substances. 

HSC Chemistry Syllabus

Investigate the processes used to analyse the structure of simple organic compounds addressed in the course, including but not limited to:

• proton and carbon-13 NMR

• mass spectrometry

• infrared spectroscopy (ACSCH130)

What is Nuclear Magnetic Resonance (NMR)?

This video introduces the concept of nuclear magnetic resonance (NMR). It explains how NMR can be used to analyse the structure of organic molecules.

 

What is NMR Spectroscopy?

• NMR spectroscopy is a quantitative technique that analyses the magnetic properties of nuclei in a molecule to provide information about the number and type of atoms in a molecule, as well as how they are connected.

• Nuclear spin – Nuclei with an odd atomic mass or atomic number have a property called nuclear spin.
  • In the absence of a magnetic field, nuclei’s spins are orientated randomly.
  • In the presence of a magnetic field, nuclear spins become parallel to the direction of the field. Nuclei whose spin are in the same direction as the magnetic field occupy a lower energy state. There are many more nuclei in the lower energy state than the higher energy state (opposite to the field’s direction)

 

• How it works – when a magnetic field is applied, radiofrequency EM radiation is blasted at a sample to change the nuclear spin from the lower energy state to higher energy state.
  • The frequency depends on both the nuclei and the strength of the magnetic field
  • EM radiation that precisely matches the change in energy will stimulate a transition between the energy two states
  • Only nuclei that possess a net magnetic spin can be detected and analysed using NMR e.g. carbon-13 and hydrogen-1 (proton).

 

• The radiofrequency required to stimulate a transition is affected by a nucleus’ local chemical environment:
  • Electrons of nearby atoms can create magnetic fields that either oppose the nucleus’ own magnetic field (shielding) or enhance the field (de-shielding)
    • Electronegative atoms e.g. O, draw electrons towards the nucleus, creating a ‘shielding’ effect and reducing the intensity of the magnetic field acting on the nucleus.
    • Atoms, whose electrons are drawn away from, experience ‘de-shielding’ effects. In contrast, these atoms experience a stronger magnetic field.

 

• Chemical environment includes both structure and geometry of an organic molecule
• The effect due to a nucleus’ unique chemical environment is represented as ‘chemical shift’ in an NMR spectrum
• The local chemical environment is the total shielding/de-shielding effects acting on an atom.