Isotropic EPR spectra of rigid diradicals (X-band, 9.5 GHz) ?

Isotropic EPR spectra of diradicals can be surprisingly complex, particularly when singlet-triplet mixing is observed. This happens when the exchange integral J (e.g., the energy difference between the singlet and triplet states) is comparable to the isotropic hyperfine constant a.

The exact line shape depends on the J/a ratio. For instance, the six-line spectrum in the simulator shows a diradical containing one I=1/2 nucleus with J = a. Try to change the value of J. For very weak coupling between the two radicals (e.g., J<<a), the spectrum contains two lines, as if the two radicals do not interact with each other at all. For very strong coupling (e.g., J>>a), the spectrum shows a 1:2:1 triplet, as though one unpaired electron interacts with two equivalent nuclei. Note however that the distance between the lines in this trpilet is a/2, not a! See more details in the Exercises section.

The diradical spectra are most commonly observed for bisnitroxides, compounds with two nitroxide radicals close to each other. When the two nitroxides are linked by a flexible (e.g., alkyl) bridge, the spectra are further complicated by the conformational dynamics. For rigid bisnitroxides, the spectral shape depends strongly on the J/a ratio. When J is similar to a, triplet-singlet mixing results in the appearance of many unexpected lines in the spectrum.

The simulator shows an extreme case with J<<a. This corresponds to extremely weakly coupled radicals, and the spectrum (1:1:1 triplet) matches the shape expected for two equivalent and not-interacting nitroxides. Click the Save this spectrum? button. Click and hold the orange up triangle next to the J value. As the exchange integral increases, extra lines appear. As the value of J becomes very large (more than 100 times greater than a), the spectrum simplifies to a 1:2:3:2:1 quintet, which would be expected for an upaired electron interacting with two equivalent I=1 nuclei. Note though the distance between the lines of the quintet (red spectrum) is half that of the initial triplet (saved orange spectrum). So the distance between adjacent lines in the diradical patterns with J>>a is a/2, not a!