Spin–Spin Coupling Constant: Overview

In bromoethane, the three methyl protons are coupled to the two methylene protons three bonds away.

So, the signal from the methyl protons is split into a triplet, while the methylene protons appear as a quartet.

Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-one-half state.

It follows that the paired electron in the hydrogen–carbon bond must have a spin of plus-one-half.

Here, the methyl and methylene protons affect adjacent carbon atoms, through which the effect becomes mutual.

Accordingly, the absorption frequencies of the peaks in coupled multiplets are separated by the same interval, which is 7.2 hertz for bromoethane.

This interval is the coupling constant, J, which is a quantitative measure of the field-independent coupling interaction.

Along with the chemical shift in ppm, integral value, and multiplicity, the coupling constant is included when reporting NMR spectra in the abbreviated form.