Not all Couplings are Equal

When protons couple to each other, they do so with a certain intensity. This is called the “coupling constant.” Coupling constants can vary from 0 Hz (which means that the protons are not coupled, even though they are neighbors) to 16 Hz. Typically, they are around 7 Hz, but many molecules contain coupling constants that vary significantly from that. So, what happens when a molecule contains a proton which is coupled to two different protons with different coupling constants? We get a different pattern as described in the diagram below.

So, if the protons are not equivalent, they can have different coupling constants and the resulting pattern will not be a triplet, but a “doublet of doublets.” Sometimes, nonequivalent protons can be on the same carbon as described on the next slide.