Micro view of NMR

NMR is an instrumental technique that uses photons of radio frequency energy to cause a transition, or change in state, in an atom. Radio is used because transitions at the atomic level are quantized, and the amount of energy needed to cause these transitions happens to fall in the radio region of the electromagnetic spectrum. Quantized energy means there has to be the right amount of energy to cause a change of state, too much or too little and no change occurs.

There are many transitions in atoms. The ones of interest in NMR are quantum spin transitions of protons and neutrons. Quantum spin in protons and neutrons have two states, which are normally equivalent, but when placed in a magnetic field they become non-equivalent, so adding the appropriate sized photon of energy can cause a transition from one to the other. This works fine for single protons and neutrons, but when these are combined into a nucleus the situation gets more complex and we have to rely on a net nuclear spin, called I.

Across the entire periodic table, nuclear spin values ranging from I = 0 to I = 8 in ½-unit increments can be found. Protons and neutrons each have net spins of ½, but this derives from the elementary quarks and gluons of which they are composed. As a result of this complexity, no simple formula exists to predict I based on the number of protons and neutrons within an atom.

For I greater than 1 there are more than two states.

List of topics in this section

• Gyromagnetic ratio