Fermi contact hyperfine splitting constant

The occurrence of hyperfine coupling for a particular isotope indicates non-zero spin density at that nucleus in accordance with the Fermi contact term (which includes the electron wave function evaluated at the nucleus) [1]. The analysis in terms of spin density then requires at least a comparison with the (calculated [5]) isotropic hyperfine splitting constant ao, ideally, results from increasingly available open-shell quantum-chemical calculation procedures [64] are employed. 

Hyperfine splitting constants are proportional to the electron spin density pg, at or near the position of any magnetic nucleus. In the following we T1 report only the Fermi Contact term at the Nitrogen nucleus, which is given by [29]... 

The hyperfine coupling tensor (A) describes the interaction between the electronic spin density and the nuclear magnetic momentum, and can be split into two terms. The first term, usually referred to as Fermi contact interaction, is an isotropic contribution also known as hyperfine coupling constant (HCC), and is related to the spin density at the corresponding nucleus n by [25]... 

The magnitudes of the hyperfine splitting parameters also yield information about the electron distribution in the molecule. The theory of the electron-nuclear coupling interaction was first worked out by Fermi, who showed that the constant a depended on the electron density at the nucleus. For a free hydrogen atom, a is given by the Fermi contact interaction in the forrn ... 

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