Chemical Exchange

Many OH and NH compounds are weak acids and weak bases and can undergo autoprotolysis, which means that a proton can be transferred from one molecule to another. Suppose we have a compound such as 2-aminoethanol, H2NCH2CH2OH. This substance normally would be expected to have an NH2 proton resonance at about 1ppm1ppm and an OH proton resonance at about 3ppm. Autoprotolysis equilibria can exchange the protons between the molecules and also from one end to the other as shown below, even if the equilibria are not very favorable.

Such equilibria can be established very rapidly, especially if traces of a strong acid or a strong base are present. In such circumstances, a single average (−NH2,−OH) proton signal is observed, because the excitation of a given proton from its lower-energy magnetic state to its higher-energy magnetic state occurs while it is partly on oxygen and partly on nitrogen. This is the same kind of chemical shift averaging that occurs for rapidly equilibrating conformations.