Illustrated Glossary of Organic Chemistry

Spin quantum number (I): The position of the nuclear spin axis relative to the applied magnetic field B0. (This is a greatly simplified, but for the purposes of an introductory study of NMR spectroscopy, a conceptually useful interpretation. A more rigorous definition involves nuclear spin angular momentum. See the Wikipedia for more detail.)

I = +1/2
Nuclear spin axis (in red) and B0 parallel
Ground state
Release energy

Add energy

I = -1/2
Nuclear spin axis (in red) and B0 antiparallel
Excited state