What is electron paramagnetic resonance spectroscopy (EPR)?

Electron paramagnetic resonance spectroscopy (EPR) is fundamentally a technique used to investigate species that have unpaired electrons, which gives rise to paramagnetism. The core principle of EPR involves the absorption of microwave radiation by these unpaired electrons in the presence of a magnetic field. When unpaired electrons are subjected to a magnetic field, they can transition between different energy levels. The EPR technique measures the resonant absorption of microwave energy corresponding to these transitions, allowing scientists to gain insights into the electronic structure and environment of the paramagnetic species being studied.

This technique is particularly useful for characterizing free radicals and transition metal complexes that contain unpaired electrons, enabling detailed analysis of their electronic configurations and interactions. The unpaired electrons are essential for EPR, as diamagnetic species, which contain paired electrons, do not produce a detectable EPR signal due to the lack of magnetic moment. This makes option B the accurate description of what EPR entails.