Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals

A free unpaired electron would ideally lead to a single peak in the EPR spectrum resulting from the transition between two spin energy states.

However, in the presence of neighboring spin-active nuclei, the spectrum shows hyperfine splitting due to coupling between the electronic spin and nuclear spin.

The number of peaks obtained is given by 2nI + 1, where n is the number of equivalent nuclei present, and I, corresponds to the nuclear spin.

Thus, a methyl radical bearing three spin-active nuclei shows four peaks in the EPR spectrum—a quartet pattern—with relative peak intensities of 1:3:3:1.

In the case of a 1,4-benzosemiquinone radical, as the unpaired electron is delocalized over the ring, it exhibits hyperfine coupling with the ring hydrogen atoms, resulting in five peaks. 

In EPR spectroscopy, the coupling intensity—known as the hyperfine splitting constant—is measured in millitesla or gauss units.

The magnitude of the splitting constant indicates the geometry of various radicals.