AX Coupling:

The alkene signals are exist as doublets since the alkene protons A and X are coupling with each other and are mutually affecting the magnetic ?eld that they experience. In figure, we have a simpli?ed diagram in which the two alkene protons are in white, separated from each other through the three bonds and the two alkene carbon atoms in black.

Figure:  AX Coupling.

 Let us focus on proton X in figure. An external magnetic ?eld Bo forces this proton to adopt two probable orientations with the more stable orientation displayed in figure. Energy having similar frequency like the Larmor frequency is now applied resultant in resonance and a signal in the nmr spectrum.

Very much for proton X, but what is happening to proton A while all this is happening? Applying a magnetic ?eld as well forces this proton to adopt two possible orientations. Though, the precessional frequency for proton A is dissimilar from that of proton X since proton A is in a dissimilar environment. Hence, applying energy that has the correct frequency to make proton X resonate will have no such type of effect on proton A. This means that the two probable orientations for proton A are long lived, and resonance does not occur between them. The dipole moments related with these orientations can thus produce a small magnetic ?eld that will affect X. So necessarily proton A can be viewed like a small magnet that has two different orientations. The two dissimilar magnetic ?elds generated by A are experienced by proton X but it is significant to realize that the effect is not transmitted by space. The nuclei are also far apart from each other for that to occur. Instead the effect is transmitted by the three bonds among the two protons. A complete explanation of how this occurs is not possible here but the entire effect is that proton A generates two equal however opposing secondary magnetic ?elds at proton X. Similar thing happens in reverse while energy is applied to resonate proton A. In that condition, proton X does not resonate but has two equal and opposite secondary magnetic effects on A.