Electron spin resonance spectroscopy

The electron spin resonance spectroscopy (or ESR) (sometimes referred to as electron paramagnetic resonance spectroscopy, or EPR) technique is similar to nuclear magnetic resonance (NMR), except that it is the electron spin[?] rather than the nuclear spin that is investigated. This means that the substance under investigation must possess at least one unpaired electron, generally meaning that it must be a free radical. Because most stable molecules have a closed-shell configuration without a suitable unpaired spin, the technique is less widely used than NMR.

The degeneracy of the spin states is lifted by the presence of a strong magnetic field, and radiation in the microwave part of the electromagnetic spectrum is used to induce transitions between spin states.