Nuclear Magnetic Resonance (NMR) Spectroscopy: An Introduction, Cheat Sheet of Chemistry

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Nuclear Magnetic Resonance (NMR)

Spectroscopy

• The nuclei of many atoms have a property called nuclear spin (I > 0).
• We can picture this as the nucleus ‘rotating’ - like the earth.
• Any spinning object has angular momentum. For sub-atomic particles (electrons, nuclei), the angular momentum is quantized, i.e. angular momentum can only have certain fixed values or more simply we can say that only certain rotation speeds are possible.
• The nuclear spin is characterized by a quantum number, I (spin quantum number).
• The nuclear spin is also associated with a magnetic

Larmor precession

• The spin axis will rotate around the direction of the field. The angle is always the same.
• Precession of the +1/2 component around the field direction. This is called Larmor precession.
• The frequency of precession is given by ωo = ϒBo. Bo is the field strength (in

Tesla) and ϒ is the magnetogyric ratio (in T -1s -1). It is the ratio of magnetic

moment to angular momentum. This has a different value for every nucleus.