Spin - spin coupling:
Let us seem at what happens to the signal for proton A while this coupling occurs. If coupling did not place, the signal would be a singlet. Though with coupling, A experiences two different secondary magnetic ?elds from X and is split into two peaks.
Figure: Spin - spin coupling.
The peak at higher chemical shift takes place when the secondary ?eld generated through proton X is aligned with the applied ?eld. The peak at lower chemical shift takes place when the secondary ?eld is against the applied ?eld. Because both effects are equally likely and of similar magnitude the signal for A is split into a doublet in which the peaks are of equal height and equally shifted from the original chemical shift. The separation among the peaks can be calculated and is called the coupling constant. It is given that the symbol J and is measured in hertz rather than ppm. The coupling is further fixed by de?ning the coupled protons. So, JAX is the coupling constant among proton A and proton X.