Question #11256

The nuclei of atoms generate very small magnetic fields, and when part of a molecule magnetic fields from one atom can have an effect on those of other atoms. The energy of adjacent nuclei therefore can changing a little (as does their frequency under resonance). The effect is relatively long range (in atomic terms anyway), and one atom's nucleus can have an effect on the energy and resonant frequency of another atom that is between 1 and 3 bonds distant from it. This effect is known as "spin-spin coupling".

It is important because it provide further detailed info about the structure of a molecule rather than just using chemical shift alone.

Imagine a proton in a molecule, which is detected as a singlet peak on a proton NMR spectrum. Now imagine that this same proton is now in a molecule where there is another proton 3 atoms away. The effect of the nucleus of this proton will cause the signal from the first proton to split into two - one peak a few Hz above that of the initial proton, and one peak the same number of Hz below that of the initial proton.
These split peaks each are of 50% of the area of the singlet peak. The difference between the frequencies of the two peaks is known as "coupling constant".

So by examining both chemical shift and coupling constant you can get a better picture of what protons are present in the molecule, and how they are bonded to (and how they are from) other protons. It allows you to get a better picture of likely structure.