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Why are unpaired electrons necessary for electron paramagnetic resonance?

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magnetic resonance Figure 1: Precession of a magnetic dipole moment μ in the presence of a constant field H and a rotating field H′ (see text)

Unpaired electrons are necessary for electron paramagnetic resonance (EPR) because they possess a net spin angular momentum and a corresponding magnetic moment, which allows them to behave like tiny magnets.12

In many atoms, electrons exist in pairs with spins that are oppositely directed, effectively neutralizing their magnetic moments and resulting in no net spin angular momentum. In contrast, atoms or molecules with one or more unpaired electrons exhibit paramagnetism, meaning they are attracted to an external magnetic field.23

When these materials are subjected to a strong, constant magnetic field, the magnetic axes of the unpaired electrons partially align with the field and precess. EPR occurs when the material absorbs energy from a weak, oscillating microwave field at a frequency that matches the natural precession frequency of these electrons. Without unpaired electrons, there would be no net magnetic moment to interact with the external field, and therefore no resonance phenomenon to detect.12