¹H - ¹H Coupling

You'll notice in the spectra that we've seen that the signals don't appear as single lines, sometimes they appear as multiple lines. This is due to ¹H - ¹H coupling (also called spin-spin splitting or J-coupling). Here's how it works: Imagine we have a molecule which contains a proton (let's call it H_A) attached to a carbon, and that this carbon is attached to another carbon which also contains a proton (let's call it H_B). It turns out that H_A feels the presence of H_B. Recall that these protons are tiny little magnets, that can be oriented either with or against the magnetic field of the NMR machine. When the field created by H_B reinforces the magnetic field of the NMR machine (B₀ ) H_A feels a slightly stronger field, but when the field created by H_B opposes B₀, H_A feels a slightly weaker field. So, we see two signals for H_A depending on the alignment of H_B. The same is true for H_B, it can feel either a slightly stronger or weaker field due to H_A's presence. So, rather than see a single line for each of these protons, we see two lines for each.