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What causes hyperfine structure in electron-spin resonance spectra?

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magnetic resonance Figure 1: Precession of a magnetic dipole moment μ in the presence of a constant field H and a rotating field H′ (see text)

In electron-spin resonance (ESR), hyperfine structure is caused by the interaction between the electronic magnetization and the magnetic moment of the nucleus.1

While fine structure in ESR spectra arises from the energy levels associated with the coupling of magnetic moments to an external magnetic field and the electrostatic energy of the ligand field, hyperfine structure represents a further splitting of these components. This interaction occurs because the electronic spin is not necessarily localized on a single atom; if the magnetization is spread over multiple atoms, it can interact with several nuclei, leading to additional splitting known as superhyperfine structure.1

In broader physical contexts, hyperfine structure is generally attributed to the magnetic interaction between the total electronic magnetic moment (orbital plus spin) and the nuclear magnetic moment, as well as the electrostatic interaction between the nucleus's electric quadrupole moment and the electron. These interactions provide a detailed mapping of electron-spin densities, which can be measured with high precision using techniques such as electron-nuclear double resonance (ENDOR).23