5. NMR exchange

Exchange is not a specific NMR phenomenon. It is a reversible molecular phenomenon. There are essentially two types of exchange: conformational and chemical. Conformational exchange involves no bond breakage, but simply a change of shape limited to a particular site or affecting the whole molecule (e.g. rotation of a methyl group around the σ bond, inversion of a cyclohexane). Chemical exchange involves breaking bonds in such a way that an atom or group of atoms moves from one site to another in the same molecule, or from one molecule to another (e.g. keto-enolic equilibrium, mobile protons bonded to hetero atoms). All atoms can be involved. In both cases, certain atoms or groups of atoms change their magnetic environment. The exchange therefore has important repercussions on the observation of parameters detected by NMR.

The kinetics of the exchange are decisive....