Chemists use a simple rule called the n plus 1 rule to predict splitting patterns. A signal splits into n plus 1 lines. Here, n is the number of equivalent neighboring hydrogen atoms it couples with.
Setting n equal to 1 gives a doublet. The neighboring hydrogen atom can point its spin in one of two directions, and both directions are nearly equally likely. This splits the parent signal into two lines of close to equal size.
The gap between the two lines is called the coupling constant, written as J and measured in hertz. J shows how strongly the two nuclei interact through the bonds connecting them. Importantly, J does not change with the strength of the spectrometer’s magnet.
This helps chemists tell a true doublet from two separate peaks that just happen to sit near each other. A true doublet is caused by coupling, not coincidence. Doublets can also show up in EPR spectra, when an unpaired electron interacts with one nearby nuclear spin.
A very large J value usually points to coupling between hydrogens on adjacent carbons within a rigid ring system. Chemists read splitting patterns like these to map out a molecule’s structure piece by piece.