The following is an excerpt from a lecture by Dr. Jeffrey H. Simpson discussing the origins of NMR signals.
Some nuclei have spin.
For a complete listing of NMR active nuclei, see our NMR Frequency Table . Nuclear spin is a natural precipitation of the Heisenberg uncertanty principle. Quantum mechanics also dictates that the nuclear spin be quantized into discreet values or states. This results from the small size of the nucleus. The angular momentum of the nucleus P can be related to the nuclear magnetic moment m via a proportionality constant g .