Energy-level splitting, electron paramagnetic resonance

Electron spin resonance (or electron paramagnetic resonance) is now a well-established analytical technique, which also offers a unique probe into the details of molecular structure. The energy levels involved are very close together and reflect essentially the properties of a single electronic state split by a small perturbation. 

The real power of electron spin resonance spectroscopy for structural studies is based on the interaction of the impaired electron spin with nuclear spins. This interaction splits the energy levels and often allows determination of the atomic or molecular structure of species containing unpaired electrons, and of the ligation scheme around paramagnetic transition-metal ions. The more complete Hamiltonian is given in equation 2 for a species containing one unpaired electron, where the summations are over all the nuclei, n, interacting with the electron spin. 

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