Nuclear Spin

Protons, neutrons and electrons have spin and you can imaginge they spin much like a top does.  Depending upon the number and the type of protons and neutrons in a nuclei there a number of possible spin states.  For most nuclei important in organic chemistry (e.g. H, ¹³C) there are two possible spins state of equal energy.  You can imagine its like the top spinning to the right or spinning to the left...two states of identical energy.  They are labelled +1/2 and -1/2.  Only when we place them into a magnetic field do we see a difference in energy between these spins states.  Roughly half line up with the field and half against the field.  There is always a slight excess that are with the field.  If you placed 1,000,000 protons in a magnetic field you get about 500,005 with the field and 499,995 against the field.  Thus there is an energy difference.  The protons aligned with the field are lower in energy than the protons against the field.  If the field is turned off the protons relax back to some random spin orientation.

In an NMR experiment we actually measure the radiation emitted as the protons relax back to a random state.

This energy difference depends on the stre