NMR coupling constant ($J$)

The frequency spacing $J$ of a first-order multiplet, expressed in hertz. For a spectrum at ${\nu }_0$ MHz, $J/\mathrm{Hz}=(\Delta \delta /\mathrm{ppm})({\nu }_0/\mathrm{MHz})$. A bond-count superscript, such as ${}^{3}J$, specifies the coupling path.

Geminal coupling is spin-spin coupling between two nuclei separated by two chemical bonds, commonly two hydrogens attached to the same carbon. It is denoted ${}^{2}J$ to distinguish it from a three-bond vicinal coupling.

Equivalent hydrogens do not split one another into separately observable multiplets in a simple first-order spectrum, even if a geminal interaction exists. Nonequivalent methylene hydrogens can show it clearly. The magnitude and sign depend on molecular structure and electronic environment, so the coupling is structural evidence rather than one fixed universal constant.

Vicinal coupling is spin-spin coupling between nuclei separated by three bonds, often hydrogens on neighboring carbons, and is denoted ${}^{3}J$. Its dependence on the intervening dihedral angle is described approximately by a Karplus relation.

This makes a measured coupling useful for estimating molecular conformations. The relationship also depends on substituents and bonding, and different angles can give similar coupling magnitudes. A single coupling therefore rarely determines a conformation uniquely; several measurements and stereochemical constraints are normally combined.