Introduction to Spin-Spin Coupling in H-NMR
Recall that protons have a magnetic field associated with them (since they have a nuclear spin), and when they are placed in a magnetic field approximately half of the protons become aligned with the field and half become aligned against the field. It is the transition between these two states that we observe in NMR.
In the simplest case we expect to see a single peak for each type of proton in a molecule. But consider what happens if a proton that we are looking at (HA) is near another nonequivalent proton (HB). In half of the molecules the HA proton will be adjacent to an HB aligned with the field and in the other half the HA proton will be adjacent to an HB aligned against the field. Thus, half the HA's in the sample will feel a slightly larger magnetic field than they would in the absence of HB and half will feel a slightly smaller magnetic field. Thus, we will observe two absorptions for the HA proton. (Of course we would also observe the same thing for HB.) This splitting of the HA resonance into two peaks is termed "spin-spin coupling" or "spin-spin splitting" and the distance between the two peaks (in Hz) is called the "coupling constant" (usually represented by the symbol J). The spin-spin coupling is transmitted through the electrons in the bonds and so depends on the bonding relationship between the two hydrogens.
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