NMR = Nuclear Magnetic Resonance

Physical Principles:

Some (but not all) nuclei, such as ¹H, ¹³C, ¹⁹F, ³¹P have nuclear spin. 

A spinning charge creates a magnetic moment, so these nuclei can be thought of as tiny magnets.

If we place these nuclei in a magnetic field, they can line up with or against the field by spinning clockwise or counter clockwise.

 

Alignment with the magnetic field is lower energy than against the magnetic field.  How much lower it is depends on the strength of the magnetic field

Note that for nuclei that don’t have spin, such as ¹²C, there is no difference in energy between alignments in a magnetic field since they are not magnets.  As such, we can’t do NMR spectroscopy on ¹²C.