Ideally, an unpaired electron shows a single peak in the EPR spectrum due to the transition between the two spin energy states. However, coupling interactions can occur between the spins of the unpaired electron and any neighboring spin-active nuclei. This hyperfine coupling results in hyperfine splitting, where the EPR signal is split into multiplets. The signals split into 2nI + 1 peaks, where n is the number of equivalent nuclei and I is the nuclear spin. These splitting patterns provide valuable information about the radical. In a methyl radical, for example, the electron spin is coupled with the three spin-active hydrogen nuclei. The EPR spectrum shows four peaks (quartet) with relative peak intensities in the ratio of 1:3:3:1.
In the case of the 1,4-benzosemiquinone radical, the unpaired electron is delocalized over the ring and oxygen atoms making all protons equivalent. The electron is coupled with the four equivalent protons, splitting the signal into five peaks with relative peak intensities in the ratio of 1:4:6:4:1. The distance between the peaks is the hyperfine coupling constant, which is measured in gauss or millitesla units. The magnitude of the coupling constant indicates the geometry of various radicals.
