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# Hyperfine coupling

Hyperfine coupling refers to the interaction between an electron in an atom or molecule and one or more atomic nuclei.

### Additional recommended knowledge

Every electron has an intrinsic magnetic moment and a spin quantum number s = ½. For hyperfine coupling to occur, an electron must interact with an atomic nucleus having a spin quantum number I $\ne$ 0.

Hyperfine interactions can be measured, among other ways, in atomic and molecular spectra and in electron paramagnetic resonance spectra of free radicals and transition-metal ions.

## Terminology

The following terminology has evolved to describe spectra:

• Fine structure refers to that part of a spectrum caused by the splitting of energy levels, and spectral lines, due to interactions among electrons in a single atom or molecule. The interactions originate in the s = 1/2 quantum number for electrons.
• Hyperfine structure refers to that part of a spectrum caused by the splitting of energy levels, and spectral lines, due to an unpaired electron interacting with a nucleus having I $\ne$ 0. The electron and nucleus (nuclei) are on the same atom or within the same molecule.
• Superhyperfine structure refers to that part of a spectrum caused by the splitting of energy levels, and spectral lines, due to an unpaired electron interacting with a nucleus having I $\ne$ 0. The electron and nucleus (nuclei) are on different atoms or different molecules.
• Spin-spin structure refers to that part of a spectrum caused by the splitting of energy levels, and spectral lines, due to interactions among nuclei having I $\ne$ 0. This phenomenon is especially important in NMR spectra

## References

• G. Herzberg, Atomic Spectra and Atomic Structure. Dover, New York, 1944. See especially chapter 5.
• M. Symons, Chemical and Biochemical Aspects of Electron-Spin Resonance Spectroscopy. Wiley, New York, 1978
• J. A. Weil, J. R. Bolton, and J. E. Wertz, Electron Paramagnetic Resonance: Elementary Theory and Practical Applications. Wiley-Interscience, New York, 2001