Accounting for the solvation energy should bridge the intrinsic energy gap.Īrguing that the MNDO method is more suitable than the AM1 method for predicting the heats of formation of five-membered nitrogenated aromatic rings, García and Vilarrasa ( 88H1803) calculated that 4-fluoroimidazole 14a (R 2 = F, R 1 = R 3 = H) is 2.5 kJ mol −1 more stable than its tautomer 14b, whereas 2,4-difluoroimidazole 14a (R 1 = R 2 = F, R 3 = H) is 2.1 kJ mol −1 less stable than 14b. These inconsistencies may be explained by the fact that tautomers 14b of 4(5)-aminoimidazole and 14a of 4(5)-nitroimidazole possess higher values of dipole moments compared to their more stable gas-phase tautomers. Both predictions are in disagreement with expectations based on Charton’s equation and the data related to basicity measurements ( Table III). The AM1 calculations of enthalpies of formation of 4(5)- aminoimidazole 14 (R 2 = NH 2, R 1 = R 3 = H) and 4(5)-nitroimidazole 14 (R 2 = NO 2, R 1 = R 3 = H) point to tautomers 14a and 14b respectively as being energetically preferred in the gas phase. From the comparison of the electronic spectra of 4(5)-phenylimidazole 14 (R 2 = Ph, R 1 = R 3 = H) and 2,4(5)-diphenylimidazole 14 (R 1 = R 2 = Ph, R 3 = H) in ethanol with those calculated by using π-electron PPP method for each of the tautomeric forms, it follows that calculations for type 14a tautomers match the experimentally observed spectra better ( 86ZC378). AM1 and PM3 calculations of the relative stabilities carried out for a series of 4(5)- substituted imidazoles 14 (R 3 = H, R 2 = H, CH 3, OH, F, NO 2, Ph) are mostly in accord with the conclusion based on the Charton’s equation. Theoretical studies of the relative stabilities of tautomers 14a and 14b were carried out mostly at the semiempirical level. The equation was found to be in a qualitative agreement with the experimental data presented in Table III. Application of Charton’s equation, K T = / = 3.2 σ m, was discussed in detail. The main conclusion on the influence of substituents in the imidazole ring on the state of the tautomeric equilibria 14a ⇌ 14b is that electron- withdrawing groups favor the 4-position, i.e., the tautomers 14a with R 2 = Hal, NO 2, and so on, are the energetically preferable species. A Position of substituents is indicated according to the structure 14a.
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