Tautomerization in the Ground Electronic State

The valence tautomerization reactions must necessarily be excited-state reactions since in the ground electronic state the equilibrium lies in favor of butadiene. The data in Figure 1 show that during irradiation in dilute solution 1,3-butadiene goes exclusively to form a mixture of the 2 valence tautomers. The small defect in the mass balance can be attributed to losses due to evaporation. 

Figure 9.23. Enol (a) - keto (b) tautomerism in o-hydroxyphenyl-(l,3,5)-triazine, indicating the number of Jt-electrons in the ground state for each ring A and B (adapted from reference 16).

The 77-SCF MO method and the Del Re procedure for u-electrons have been applied by Danilov148 to evaluate the enthalpy changes on tautomeric conversion 2 -> 6 in the ground state and the first excited singlet state for cytosine and its anionic and cationic forms. The results show that although in the ground state of cytosine the tautomeric... 

An integral part of the tautomerization reaction is the electronic rearrangement. Although ionic states may be involved in intermediate steps in the reaction, in particular in the ground-state tautomerization of acetaldehyde-type compounds, usually the reactant and product have the same charge. But, as we have seen, the dipole moment can change considerably. This is at least partly due to a change in the electron distribution, and as such that is susceptible to an influence of the environment. 

In the application of theoretical studies to the azole field many of these have attempted to achieve comparisons within the range of azole molecules. Thus, calculations of electron densities, dipole moments, and energies of formation give values that reflect the decrease in azole stability as the number of nitrogen atoms increase. ° Good correlations between a and total electron densities and and chemical shifts have been obtained. " Calculations (SCF) of n-electron distributions for the ground state of imidazole do not take into account the tautomeric equivalence of the 4-and 5-positions, but predict the order of electrophilic substitution as 5 > 2 > 4242,243 Various other quantum-mechanical calculations have... 

---