How do ligand fields affect electron-spin resonance spectra?

In electron-spin resonance (ESR), ligand fields significantly influence the observed spectra by interacting with the electronic shells of magnetic centers, such as transition metal ions. Because ESR is highly sensitive to the local environment, the ligand field acts as a primary factor in determining the resonance frequencies and the resulting spectral structure.1

The effects of ligand fields on ESR spectra include:

• Fine Structure: The electrostatic energy between the electronic shells and the ligand field contributes to the total energy of the magnetic center. This interaction, combined with the coupling between magnetic moments and an external magnetic field, creates a spectrum characterized by multiple resonance frequencies known as fine structure.1
• Orbital Momentum Reduction: Ligand fields can reduce the magnetism caused by orbital motion. The extent of this reduction depends on the relative strength of the ligand field versus spin-orbit coupling. For example, in the iron group, ligand fields are typically stronger than spin-orbit coupling, leading to a strong reduction of orbital momentum, whereas in lanthanoids, the weaker ligand field leaves orbital momentum largely unreduced.1
• Hyperfine and Superhyperfine Structure: Ligand fields influence the distribution of electronic magnetization. When this magnetization spreads over multiple atoms, it interacts with nuclear moments, causing hyperfine splitting. Further interactions with neighboring nuclei result in superhyperfine structure, which provides evidence of covalent bonding between the metal ion and its ligands.1