Illustrated Glossary of Organic Chemistry

Nuclear spin state: Because it is a charged particle in motion (spinning), an atomic nucleus creates a magnetic field. The "north pole" of this spin-generated magnetic field points a certain direction in space. The nuclear spin state is the orientation of the spin-generated magnetic field relative to an external magnetic field (B₀). Think of the nucleus as a top, and the magnetic field orientation as the spin axis. This is a significant simplification. A top follows Newtonian mechanics whereas a nucleus obeys quantum mechanics. However for an introductory discussion of NMR spectroscopy theory, the simplification is useful.

A proton has two nuclear spin states: I = +1/2 (spin-generated magnetic field parallel to B₀; the nuclear spin ground state) and I = -1/2 (spin-generated magnetic field antiparallel to B₀; the nuclear spin excited state).