Tautomerism is a chemical equilibrium which involves rapid transference of
a hydrogen atom. Its importance in biochemistry, medicinal chemistry, pharmacology
and organic synthesis, as well as the wide variety of molecules in which it occurs,
makes it an interesting chemical issue to be studied. NMR bears the advantage of
allowing equilibrium observation without shifting it. The aim of this work is to sum up
a variety of experiments that can be carried out on tautomeric equilibria in order to
obtain structural and mechanistic information. In every case, the two (or more) major
tautomeric forms must have relatively low conversion rates, i.e., they must exist long
enough to survive (in average) the NMR experiment and then show different but
overlapped NMR spectra. Assignation of the peaks to their corresponding tautomeric
form has to be done with regarding signal integration, multiplicity and chemical shift of
the signals. Theoretical calculations might be carried out in order to do this assignation.
Once found two (or more) independent and non-overlapped peaks corresponding to
each tautomer, their integration permits tautomeric contents and tautomerization
constants calculation. Herein, equilibrium shifts caused by the presence of substituents
(causing electronic and steric effects), solvents (interacting in different ways with the
tautomers), internal chemical interactions (such as hydrogen bonds), tautomer-tautomer
interactions (producing the formation of dimers) and temperature variation are
discussed using a variety of compounds, such as ketonitriles, ketoamides and
salicylaldimines, among others. All these facts give information about the causes of the
stabilization or destabilization of different tautomeric forms.
Keywords: NMR, Solvent effect, Substituent effect, Tautomerism, Theoretical
calculations.
