Other Types of Exchange Tautomerism

The exchange phenomena that have been presented thus far in this chapter have been essentially intermolecular in nature. They are examples of dynamic NMR, in which the NMR spectrometer is used to study processes that involve the rapid interconversion of molecular species. The rates of these interconversions as a function of temperature can be studied, and they can be compared with the NMR time-scale. 

Molecules with structures that differ markedly in the arrangement of atoms but that exist in equilibrium with one another are called tautomers. The most common type of tautomerism is keto-enol tautomerism, in which the species differ mainly by the position of a hydrogen atom. 

In general, the keto form is much more stable than the enol form, and the equilibrium lies strongly in favor of the keto form. Keto-enol tautomerism is generally considered an intermolecu-lar process. 1,3-Dicarbonyl compounds are capable of exhibiting keto-enol tautomerism this is 

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FIGURE 8.7 H NMR spectrum of acetylacetone. The O—H proton of the enol tautomer is not shown. 

By comparing the integrals of the two different methyl peaks, one can easily calculate the equilibrium distribution of the two tautomers. 

Another type of tautomerism, intramolecular in nature, is called valence tautomerism (or valence isomerization). Valence tautomers rapidly interconvert with one another, but the tautomeric forms differ principally by the positions of covalent bonds rather than by the positions of protons. There are many examples of valence tautomerism in the literature. An interesting example is the isomerization of bullvalene, an interesting compound with threefold symmetry. At low temperatures (below -85°C), the proton NMR spectrum of bullvalene consists of four complex multiplets (each 

3-Dicarbonyl compounds are capable of exhibiting keto-enol tautomerism this is illustrated for the case of acetylacetone. 

For most 1,3-dicarbonyl compounds, the equilibrium lies substantially to the right, favoring the enol. The enol form is stabilized through the formation of a strong intramolecular hydrogen bond. 

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There are two mutation processes where mispairing due to enol/imino tautomeric forms could be involved. In one, transition, a purine is replaced by another purine or a pyrimidine by another pyrimidine in the other, transversion, a purine/pyrimidine or pyrimidine/purine exchange takes place. In these mutations, formation of base pairs of the type illustrated in Fig. 20.7 is postulated. 

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