Origin of Splitting in NMR Spectra
This page describes the reason that you get clusters of peaks in a high resolution NMR spectrum in place of simple peaks in the low-resolution spectrum. The effect is known as spin-spin coupling or spin-spin splitting.
Note: If you have come straight to this page via a search engine, you should realise that it is only a "footnote" to a page on high-resolution NMR spectra.
The Origin of a Doublet
Consider what the high resolution NMR spectrum of the compound CH2Cl-CHCl2 would look like.
Note: I'm sorry about the absence of a real spectrum. The SDBS database didn't have a suitable NMR spectrum for this compound.
Focus on the CH2 group. Why is that a doublet?
Remember that the peaks in an NMR spectrum are in different places because the hydrogens are experiencing different magnetic fields due to their different environments.
Two peaks close together must mean that those particular hydrogens are experiencing two slightly different magnetic fields. Those two slightly different fields are caused by the hydrogen in the CH group next door.
The hydrogen next door has a small magnetic field of its own, which could be aligned with the external magnetic field or opposed to it. Depending on which way around it is aligned, it will either strengthen or weaken the field felt by the CH2 hydrogens.
There is an equal chance of either of these arrangements happening and so there will be two peaks due to the CH2 hydrogens, close together and with equal areas under them (because of the 50/50 chance of either arrangement).
The Origin of a Triplet
Now focus on the CH group in the compound CH2Cl-CHCl2. Why is that a triplet? It must be a triplet because that hydrogen is experiencing any one of three slightly different magnetic fields.
Think about the magnetic alignments of the hydrogens on the next door CH2 group. These are the various possibilities:
The two arrangements in the centre of the diagram produce the same field (exactly the same as the external field). So there are three possible magnetic fields that the CH hydrogen could feel, and so there are three peaks close together – a triplet.
The areas under the peaks are in the ratio of 1:2:1 because that represents the chances of these various magnetic fields occurring.
The Origin of a Quartet
If you apply the same sort of argument to hydrogens next door to a CH3 group, you will find that they could be experiencing any one of four different magnetic fields depending on the alignment of the CH3 hydrogens.
All the arrangements in the second line produce the same field. All the alignments in the third line also produce the same field, but this time a bit smaller. There are four different possible fields, with the chances of them arising in the ratio 1:3:3:1.
So a CH3 group produces a quartet in the spectrum of the hydrogens of the next door group, with the peak sizes in the ratio 1:3:3:1.